lint(metrics/family): add is_empty()

```shell
$ cargo clippy --all-features --all-targets --tests
   Compiling prometheus-client v0.23.0 (/path/to/prometheus-client)
warning: struct `Family` has a public `len` method, but no `is_empty` method
   --> src/metrics/family.rs:309:5
    |
309 |     pub fn len(&self) -> usize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty
    = note: `#[warn(clippy::len_without_is_empty)]` on by default

warning: `prometheus-client` (lib) generated 1 warning
warning: `prometheus-client` (lib test) generated 1 warning (1 duplicate)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.63s
```

Signed-off-by: katelyn martin <[email protected]>

src/metrics/family.rs

@@ -310,6 +310,11 @@ impl<S: Clone + std::hash::Hash + Eq, M, C: MetricConstructor<M>> Family<S, M, C
         self.metrics.read().len()
     }
 
+    /// Returns `true` if the family contains no metrics.
+    pub fn is_empty(&self) -> bool {
+        self.metrics.read().is_empty()
+    }
+
     pub(crate) fn read(&self) -> RwLockReadGuard<HashMap<S, M>> {
         self.metrics.read()
     }