fix(datadog metrics sink): fix metric type comparison using wrong operand in sort

changelog.d/25621_datadog_metrics_sort.fix.md

@@ -0,0 +1,3 @@
+Fixed a bug in the `datadog_metrics` sink where the metric type name was compared against itself (instead of the peer metric) when sorting metrics before encoding. The sort key is `(type_name, metric_name, timestamp)`, but the type comparison was a no-op, making `metric_name` the effective primary key. The fix restores the intended ordering.
+
+authors: gwenaskell

src/sinks/datadog/metrics/sink.rs

@@ -217,7 +217,7 @@ fn sort_and_collapse_counters_by_series_and_timestamp(mut metrics: Vec<Metric>)
             a.timestamp().map(|dt| dt.timestamp()).unwrap_or(now_ts),
         )
             .cmp(&(
-                a.value().as_name(),
+                b.value().as_name(),
                 b.series(),
                 b.timestamp().map(|dt| dt.timestamp()).unwrap_or(now_ts),
             ))
@@ -393,6 +393,24 @@ mod tests {
         assert_eq!(expected, actual);
     }
 
+    #[test]
+    fn sort_metric_type_is_primary_sort_key() {
+        // The sort key is (type_name, series, timestamp). "counter" < "gauge" alphabetically,
+        // so a counter must always precede a gauge regardless of series name.
+        let input = vec![
+            create_gauge("aaa", 1.0),
+            create_counter("zzz", 1.0),
+            create_counter("aaa", 1.0),
+        ];
+        let expected = vec![
+            create_counter("aaa", 1.0),
+            create_counter("zzz", 1.0),
+            create_gauge("aaa", 1.0),
+        ];
+        let actual = sort_and_collapse_counters_by_series_and_timestamp(input);
+        assert_eq!(expected, actual);
+    }
+
     #[test]
     fn collapse_identical_metrics_multiple_timestamps() {
         let ts_1 = Utc::now() - Duration::from_secs(5);