feat(metrics/family): 🍭 len() returns the number of metrics

CHANGELOG.md

@@ -15,10 +15,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Filter out empty metric families, to match the go client. See [PR 279].
 - `Histogram` now exposes `count()` and `sum()` methods when the `test-util`
   feature is enabled. See [PR 242].
+- `Family` now exposes `len()` and `is_empty()` methods when the
+  `test-util` feature is enabled. See [PR 246].
 
 [PR 279]: https://github.com/prometheus/client_rust/pull/279
 [PR 281]: https://github.com/prometheus/client_rust/pull/281
 [PR 242]: https://github.com/prometheus/client_rust/pull/242
+[PR 246]: https://github.com/prometheus/client_rust/pull/246
 
 ## [0.24.0]
 

src/metrics/family.rs

@@ -341,6 +341,35 @@ impl<S: Clone + std::hash::Hash + Eq, M, C: MetricConstructor<M>> Family<S, M, C
         self.metrics.write().clear()
     }
 
+    /// Returns the number of metrics in this family.
+    ///
+    /// ```
+    /// # use prometheus_client::metrics::counter::{Atomic, Counter};
+    /// # use prometheus_client::metrics::family::Family;
+    /// #
+    /// let family = Family::<Vec<(String, String)>, Counter>::default();
+    /// assert_eq!(family.len(), 0);
+    ///
+    /// // Will create the metric with label `method="GET"` on first call and
+    /// // return a reference.
+    /// family.get_or_create(&vec![("method".to_owned(), "GET".to_owned())]).inc();
+    /// assert_eq!(family.len(), 1);
+    ///
+    /// // Clear the family of all label sets.
+    /// family.clear();
+    /// assert_eq!(family.len(), 0);
+    /// ```
+    #[cfg(any(test, feature = "test-util"))]
+    pub fn len(&self) -> usize {
+        self.metrics.read().len()
+    }
+
+    /// Returns `true` if the family contains no metrics.
+    #[cfg(any(test, feature = "test-util"))]
+    pub fn is_empty(&self) -> bool {
+        self.metrics.read().is_empty()
+    }
+
     pub(crate) fn read(&self) -> RwLockReadGuard<'_, HashMap<S, M>> {
         self.metrics.read()
     }