feat(metrics): add Summary implementation

This commit introduces the `Summary` metric type for the Rust Prometheus client, addressing the requirements outlined in prometheus#40.

### Key Highlights:
- Implements the `Summary` type to represent summary statistics such as sum, count, and quantiles.
- The implementation does not include a specific quantile calculation algorithm. Users are expected to provide precomputed quantiles based on their chosen algorithm.
- Supports encoding summary metrics following the OpenMetrics standard.

### Details:
- `Summary` provides:
  - `reset` method to update its internal state with sum, count, and quantiles.
  - Integration with the `TypedMetric` and `EncodeMetric` traits to support encoding and registration with the Prometheus client.

- Documentation and tests have been added to ensure correct usage and behavior.
- This implementation is designed to give users flexibility in handling quantile computation.

### Notes:
- Future work could explore integrating popular quantile calculation algorithms to offer built-in support while retaining the option for custom implementations.

src/encoding.rs

@@ -4,6 +4,7 @@ pub use prometheus_client_derive_encode::*;
 
 use crate::metrics::exemplar::Exemplar;
 use crate::metrics::MetricType;
+use crate::metrics::summary::Numeric;
 use crate::registry::{Prefix, Unit};
 use std::borrow::Cow;
 use std::collections::HashMap;
@@ -186,6 +187,21 @@ impl MetricEncoder<'_> {
         )
     }
 
+    /// Encode a summary.
+    pub fn encode_summary<T: ToString + Numeric>(
+        &mut self,
+        sum: f64,
+        count: u64,
+        quantiles: &[(f64, T)],
+    ) -> Result<(), std::fmt::Error> {
+        for_both_mut!(
+            self,
+            MetricEncoderInner,
+            e,
+            e.encode_summary(sum, count, quantiles)
+        )
+    }
+
     /// Encode a metric family.
     pub fn encode_family<'s, S: EncodeLabelSet>(
         &'s mut self,

src/encoding/protobuf.rs

@@ -33,6 +33,7 @@ pub mod openmetrics_data_model {
 use std::{borrow::Cow, collections::HashMap};
 
 use crate::metrics::MetricType;
+use crate::metrics::summary::Numeric;
 use crate::registry::{Registry, Unit};
 use crate::{metrics::exemplar::Exemplar, registry::Prefix};
 
@@ -53,6 +54,7 @@ impl From<MetricType> for openmetrics_data_model::MetricType {
             MetricType::Counter => openmetrics_data_model::MetricType::Counter,
             MetricType::Gauge => openmetrics_data_model::MetricType::Gauge,
             MetricType::Histogram => openmetrics_data_model::MetricType::Histogram,
+            MetricType::Summary => openmetrics_data_model::MetricType::Summary,
             MetricType::Info => openmetrics_data_model::MetricType::Info,
             MetricType::Unknown => openmetrics_data_model::MetricType::Unknown,
         }
@@ -288,6 +290,41 @@ impl MetricEncoder<'_> {
 
         Ok(())
     }
+
+    pub fn encode_summary<T: Numeric>(
+        &mut self,
+        sum: f64,
+        count: u64,
+        quantiles: &[(f64, T)],
+    ) -> Result<(), std::fmt::Error> {
+        let quantiles = quantiles.iter()
+            .map(|(q, v)| {
+                openmetrics_data_model::summary_value::Quantile {
+                    quantile: *q,
+                    value: v.as_f64(),
+                }
+            })
+            .collect::<Vec<_>>();
+
+        self.family.push(openmetrics_data_model::Metric {
+            labels: self.labels.clone(),
+            metric_points: vec![openmetrics_data_model::MetricPoint {
+                value: Some(openmetrics_data_model::metric_point::Value::SummaryValue(
+                    openmetrics_data_model::SummaryValue {
+                        count,
+                        created: None,
+                        quantile: quantiles,
+                        sum: Some(openmetrics_data_model::summary_value::Sum::DoubleValue(
+                            sum,
+                        )),
+                    },
+                )),
+                ..Default::default()
+            }],
+        });
+
+        Ok(())
+    }
 }
 
 impl<S: EncodeLabelSet, V: EncodeExemplarValue> TryFrom<&Exemplar<S, V>>
@@ -448,6 +485,7 @@ mod tests {
     use crate::metrics::family::Family;
     use crate::metrics::gauge::Gauge;
     use crate::metrics::histogram::{exponential_buckets, Histogram};
+    use crate::metrics::summary::Summary;
     use crate::metrics::info::Info;
     use crate::registry::Unit;
     use std::borrow::Cow;
@@ -817,6 +855,93 @@ mod tests {
         }
     }
 
+    #[test]
+    fn encode_summary() {
+        let mut registry = Registry::default();
+        let summary = Summary::default();
+        registry.register("my_summary", "My Summary", summary.clone());
+        let quantiles = vec![
+            (0.5, 100.1),
+            (0.9, 300.2),
+            (0.99, 700.3),
+        ];
+        let _ = summary.reset(123.0, 10, quantiles);
+
+        let metric_set = encode(&registry).unwrap();
+
+        let family = metric_set.metric_families.first().unwrap();
+        assert_eq!("my_summary", family.name);
+        assert_eq!("My Summary.", family.help);
+        assert_eq!(
+            openmetrics_data_model::MetricType::Summary as i32,
+            extract_metric_type(&metric_set)
+        );
+        match extract_metric_point_value(&metric_set) {
+            openmetrics_data_model::metric_point::Value::SummaryValue(value) => {
+                assert_eq!(
+                    Some(openmetrics_data_model::summary_value::Sum::DoubleValue(
+                        123.0
+                    )),
+                    value.sum
+                );
+                assert_eq!(10, value.count);
+                assert_eq!(3, value.quantile.len());
+            }
+            _ => panic!("wrong value type"),
+        }
+    }
+
+    #[test]
+    fn encode_summary_family() {
+        let mut registry = Registry::default();
+        let family =
+            Family::new_with_constructor(|| Summary::default());
+        registry.register("my_summary", "My Summary", family.clone());
+        let quantiles = vec![
+            (0.5, 100_u64),
+            (0.9, 300),
+            (0.99, 700),
+        ];
+        let _ = family
+            .get_or_create(&vec![
+                ("method".to_string(), "POST".to_string()),
+                ("status".to_string(), "200".to_string()),
+            ])
+            .reset(123.0, 10, quantiles);
+
+        let metric_set = encode(&registry).unwrap();
+
+        let family = metric_set.metric_families.first().unwrap();
+        assert_eq!("my_summary", family.name);
+        assert_eq!("My Summary.", family.help);
+        assert_eq!(
+            openmetrics_data_model::MetricType::Summary as i32,
+            extract_metric_type(&metric_set)
+        );
+
+        let metric = family.metrics.first().unwrap();
+        assert_eq!(2, metric.labels.len());
+        assert_eq!("method", metric.labels[0].name);
+        assert_eq!("POST", metric.labels[0].value);
+        assert_eq!("status", metric.labels[1].name);
+        assert_eq!("200", metric.labels[1].value);
+
+        match extract_metric_point_value(&metric_set) {
+            openmetrics_data_model::metric_point::Value::SummaryValue(value) => {
+                assert_eq!(None, value.created);
+                assert_eq!(
+                    Some(openmetrics_data_model::summary_value::Sum::DoubleValue(
+                        123.0
+                    )),
+                    value.sum
+                );
+                assert_eq!(10, value.count);
+                assert_eq!(3, value.quantile.len());
+            }
+            _ => panic!("wrong value type"),
+        }
+    }
+
     #[test]
     fn encode_family_counter_histogram() {
         let mut registry = Registry::default();

src/encoding/text.rs

@@ -462,6 +462,41 @@ impl<'a> MetricEncoder<'a> {
         Ok(())
     }
 
+    pub fn encode_summary<T: ToString>(
+        &mut self,
+        sum: f64,
+        count: u64,
+        quantiles: &[(f64, T)],
+    ) -> Result<(), std::fmt::Error> {
+        self.write_prefix_name_unit()?;
+        self.write_suffix("sum")?;
+        self.encode_labels::<NoLabelSet>(None)?;
+        self.writer.write_str(" ")?;
+        self.writer.write_str(dtoa::Buffer::new().format(sum))?;
+        self.newline()?;
+
+        self.write_prefix_name_unit()?;
+        self.write_suffix("count")?;
+        self.encode_labels::<NoLabelSet>(None)?;
+        self.writer.write_str(" ")?;
+        self.writer.write_str(itoa::Buffer::new().format(count))?;
+        self.newline()?;
+
+        for (quantile, value) in quantiles {
+            self.write_prefix_name_unit()?;
+
+            self.encode_labels(Some(&[("quantile", *quantile)]))?;
+
+            self.writer.write_str(" ")?;
+            self.writer
+                .write_str(&value.to_string())?;
+
+            self.newline()?;
+        }
+
+        Ok(())
+    }
+
     fn newline(&mut self) -> Result<(), std::fmt::Error> {
         self.writer.write_str("\n")
     }
@@ -730,6 +765,7 @@ mod tests {
     use crate::metrics::family::Family;
     use crate::metrics::gauge::Gauge;
     use crate::metrics::histogram::{exponential_buckets, Histogram};
+    use crate::metrics::summary::Summary;
     use crate::metrics::info::Info;
     use crate::metrics::{counter::Counter, exemplar::CounterWithExemplar};
     use pyo3::{prelude::*, types::PyModule};
@@ -981,6 +1017,103 @@ mod tests {
         parse_with_python_client(encoded);
     }
 
+    #[test]
+    fn encode_summary() {
+        let mut registry = Registry::default();
+        let summary = Summary::default();
+        registry.register("my_summary", "My Summary", summary.clone());
+        let quantiles = vec![
+            (0.5, 100.1),
+            (0.9, 300.2),
+            (0.99, 700.3),
+        ];
+        let _ = summary.reset(123.0, 10, quantiles);
+
+        let mut encoded = String::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP my_summary My Summary.\n".to_owned()
+            + "# TYPE my_summary summary\n"
+            + "my_summary_sum 123.0\n"
+            + "my_summary_count 10\n"
+            + "my_summary{quantile=\"0.5\"} 100.1\n"
+            + "my_summary{quantile=\"0.9\"} 300.2\n"
+            + "my_summary{quantile=\"0.99\"} 700.3\n"
+            + "# EOF\n";
+
+        assert_eq!(expected, encoded);
+    }
+
+    #[test]
+    fn encode_summary_family() {
+        let mut registry = Registry::default();
+        let family =
+            Family::new_with_constructor(|| Summary::default());
+        registry.register("my_summary", "My Summary", family.clone());
+        let quantiles = vec![
+            (0.5, 100_u64),
+            (0.9, 300),
+            (0.99, 700),
+        ];
+        let _ = family
+            .get_or_create(&vec![
+                ("method".to_string(), "GET".to_string()),
+                ("status".to_string(), "200".to_string()),
+            ])
+            .reset(123.0, 10, quantiles);
+
+        let mut encoded = String::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP my_summary My Summary.\n".to_owned()
+            + "# TYPE my_summary summary\n"
+            + "my_summary_sum{method=\"GET\",status=\"200\"} 123.0\n"
+            + "my_summary_count{method=\"GET\",status=\"200\"} 10\n"
+            + "my_summary{quantile=\"0.5\",method=\"GET\",status=\"200\"} 100\n"
+            + "my_summary{quantile=\"0.9\",method=\"GET\",status=\"200\"} 300\n"
+            + "my_summary{quantile=\"0.99\",method=\"GET\",status=\"200\"} 700\n"
+            + "# EOF\n";
+
+        assert_eq!(expected, encoded);
+    }
+
+    #[test]
+    fn encode_summary_family_with_empty_struct_family_labels() {
+        let mut registry = Registry::default();
+        let family =
+            Family::new_with_constructor(|| Summary::default());
+        registry.register("my_summary", "My Summary", family.clone());
+
+        #[derive(Eq, PartialEq, Hash, Debug, Clone)]
+        struct EmptyLabels {}
+
+        impl EncodeLabelSet for EmptyLabels {
+            fn encode(&self, _encoder: crate::encoding::LabelSetEncoder) -> Result<(), Error> {
+                Ok(())
+            }
+        }
+
+        let quantiles = vec![
+            (0.5, 100.1),
+            (0.9, 300.2),
+            (0.99, 700.3),
+        ];
+        let _ = family.get_or_create(&EmptyLabels {}).reset(123.0, 10, quantiles);
+        let mut encoded = String::new();
+        encode(&mut encoded, &registry).unwrap();
+
+        let expected = "# HELP my_summary My Summary.\n".to_owned()
+            + "# TYPE my_summary summary\n"
+            + "my_summary_sum{} 123.0\n"
+            + "my_summary_count{} 10\n"
+            + "my_summary{quantile=\"0.5\"} 100.1\n"
+            + "my_summary{quantile=\"0.9\"} 300.2\n"
+            + "my_summary{quantile=\"0.99\"} 700.3\n"
+            + "# EOF\n";
+
+        assert_eq!(expected, encoded);
+    }
+
     #[test]
     fn sub_registry_with_prefix_and_label() {
         let top_level_metric_name = "my_top_level_metric";

src/metrics.rs

@@ -5,6 +5,7 @@ pub mod exemplar;
 pub mod family;
 pub mod gauge;
 pub mod histogram;
+pub mod summary;
 pub mod info;
 
 /// A metric that is aware of its Open Metrics metric type.
@@ -20,13 +21,13 @@ pub enum MetricType {
     Counter,
     Gauge,
     Histogram,
+    Summary,
     Info,
     Unknown,
     // Not (yet) supported metric types.
     //
     // GaugeHistogram,
     // StateSet,
-    // Summary
 }
 
 impl MetricType {
@@ -36,6 +37,7 @@ impl MetricType {
             MetricType::Counter => "counter",
             MetricType::Gauge => "gauge",
             MetricType::Histogram => "histogram",
+            MetricType::Summary => "summary",
             MetricType::Info => "info",
             MetricType::Unknown => "unknown",
         }