feat(metrics/histogram): count() and sum() accessors

fixes prometheus#241.

this commit introduces two new public methods to `Histogram`; `sum()`
and `count()` return the sum of all observations and the number of
observations made, respectively.

Signed-off-by: katelyn martin <me+cratelyn@katelyn.world>

src/metrics/histogram.rs

@@ -76,6 +76,16 @@ impl Histogram {
         self.observe_and_bucket(v);
     }
 
+    /// Returns the current sum of all observations.
+    pub fn sum(&self) -> f64 {
+        self.inner.read().sum
+    }
+
+    /// Returns the current number of observations.
+    pub fn count(&self) -> u64 {
+        self.inner.read().count
+    }
+
     /// Observes the given value, returning the index of the first bucket the
     /// value is added to.
     ///
@@ -166,4 +176,53 @@ mod tests {
             linear_buckets(0.0, 1.0, 10).collect::<Vec<_>>()
         );
     }
+
+    /// Checks that [`Histogram::count()`] works properly.
+    #[test]
+    fn count() {
+        let histogram = Histogram::new([1.0_f64, 2.0, 3.0, 4.0, 5.0].into_iter());
+        assert_eq!(
+            histogram.count(),
+            0,
+            "histogram has zero observations when instantiated"
+        );
+
+        histogram.observe(1.0);
+        assert_eq!(histogram.count(), 1, "histogram has one observation");
+
+        histogram.observe(2.5);
+        assert_eq!(histogram.count(), 2, "histogram has two observations");
+
+        histogram.observe(6.0);
+        assert_eq!(histogram.count(), 3, "histogram has three observations");
+    }
+
+    /// Checks that [`Histogram::sum()`] works properly.
+    #[test]
+    fn sum() {
+        const BUCKETS: [f64; 3] = [10.0, 100.0, 1000.0];
+        let histogram = Histogram::new(BUCKETS.into_iter());
+        assert_eq!(
+            histogram.sum(),
+            0.0,
+            "histogram sum is zero when instantiated"
+        );
+
+        histogram.observe(3.0); // 3 + 4 + 15 + 101 = 123
+        histogram.observe(4.0);
+        histogram.observe(15.0);
+        histogram.observe(101.0);
+        assert_eq!(
+            histogram.sum(),
+            123.0,
+            "histogram sum records accurate sum of observations"
+        );
+
+        histogram.observe(1111.0);
+        assert_eq!(
+            histogram.sum(),
+            1234.0,
+            "histogram sum records accurate sum of observations"
+        );
+    }
 }