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

this commit introduces a `len()` method to `Family<S, M, C>`, which
returns the number of series within a metric family.

see also #245, which allows callers to check if a family `contains()` a
given label set.

```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 <git@katelyn.world>

Author: cratelyn

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()
     }