chore: fix histogram type documentation (open-telemetry#1427)

Co-authored-by: Valentin Marchaud <[email protected]>
Co-authored-by: Daniel Dyla <[email protected]>

packages/opentelemetry-metrics/src/export/types.ts

@@ -53,21 +53,19 @@ export interface Distribution {
 
 export interface Histogram {
   /**
-   * Buckets are implemented using two different array:
-   *  - boundaries contains every boundary (which are upper boundary for each slice)
-   *  - counts contains count of event for each slice
+   * Buckets are implemented using two different arrays:
+   *  - boundaries: contains every finite bucket boundary, which are inclusive lower bounds
+   *  - counts: contains event counts for each bucket
    *
-   * Note that we'll always have n+1 (where n is the number of boundaries) slice
-   * because we need to count event that are above the highest boundary. This is the
-   * reason why it's not implement using array of object, because the last slice
-   * dont have any boundary.
+   * Note that we'll always have n+1 buckets, where n is the number of boundaries.
+   * This is because we need to count events that are below the lowest boundary.
    *
-   * Example if we measure the values: [5, 30, 5, 40, 5, 15, 15, 15, 25]
+   * Example: if we measure the values: [5, 30, 5, 40, 5, 15, 15, 15, 25]
    *  with the boundaries [ 10, 20, 30 ], we will have the following state:
    *
    * buckets: {
    *	boundaries: [10, 20, 30],
-   *	counts: [3, 3, 2, 1],
+   *	counts: [3, 3, 1, 2],
    * }
    */
   buckets: {

packages/opentelemetry-metrics/test/export/aggregators/Histogram.test.ts

@@ -72,6 +72,17 @@ describe('HistogramAggregator', () => {
       assert.equal(point.buckets.counts[1], 0);
       assert.equal(point.buckets.counts[2], 1);
     });
+
+    it('should update the third bucket since boundaries are inclusive lower bounds', () => {
+      const aggregator = new HistogramAggregator([100, 200]);
+      aggregator.update(200);
+      const point = aggregator.toPoint().value as Histogram;
+      assert.equal(point.count, 1);
+      assert.equal(point.sum, 200);
+      assert.equal(point.buckets.counts[0], 0);
+      assert.equal(point.buckets.counts[1], 0);
+      assert.equal(point.buckets.counts[2], 1);
+    });
   });
 
   describe('.count', () => {