From 3735506d24f885057462663f1f22a4345f67a28b Mon Sep 17 00:00:00 2001
From: Aaron Abbott <aaronabbott@google.com>
Date: Wed, 21 Jun 2023 20:50:49 +0000
Subject: [PATCH] fix(sdk-metrics): preserve startTime for cumulative
 ExponentialHistograms

---
 CHANGELOG.md                                  |  1 +
 .../src/aggregator/ExponentialHistogram.ts    | 33 ++++++++++---------
 .../aggregator/ExponentialHistogram.test.ts   |  9 +++++
 3 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 819fab8972..a162eae0a4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ For experimental package changes, see the [experimental CHANGELOG](experimental/
 ### :bug: (Bug Fix)
 
 * fix(sdk-metrics): Update default Histogram's boundary to match OTEL's spec [#3893](https://github.com/open-telemetry/opentelemetry-js/pull/3893/) @chigia001
+* fix(sdk-metrics): preserve startTime for cumulative ExponentialHistograms [#3934](https://github.com/open-telemetry/opentelemetry-js/pull/3934/) @aabmass
 
 ### :books: (Refine Doc)
 
diff --git a/packages/sdk-metrics/src/aggregator/ExponentialHistogram.ts b/packages/sdk-metrics/src/aggregator/ExponentialHistogram.ts
index 5a70a478a8..51c71ee211 100644
--- a/packages/sdk-metrics/src/aggregator/ExponentialHistogram.ts
+++ b/packages/sdk-metrics/src/aggregator/ExponentialHistogram.ts
@@ -213,32 +213,33 @@ export class ExponentialHistogramAccumulation implements Accumulation {
   }
 
   /**
-   * merge combines data from other into self
-   * @param {ExponentialHistogramAccumulation} other
+   * merge combines data from previous value into self
+   * @param {ExponentialHistogramAccumulation} previous
    */
-  merge(other: ExponentialHistogramAccumulation) {
+  merge(previous: ExponentialHistogramAccumulation) {
     if (this._count === 0) {
-      this._min = other.min;
-      this._max = other.max;
-    } else if (other.count !== 0) {
-      if (other.min < this.min) {
-        this._min = other.min;
+      this._min = previous.min;
+      this._max = previous.max;
+    } else if (previous.count !== 0) {
+      if (previous.min < this.min) {
+        this._min = previous.min;
       }
-      if (other.max > this.max) {
-        this._max = other.max;
+      if (previous.max > this.max) {
+        this._max = previous.max;
       }
     }
 
-    this._sum += other.sum;
-    this._count += other.count;
-    this._zeroCount += other.zeroCount;
+    this.startTime = previous.startTime;
+    this._sum += previous.sum;
+    this._count += previous.count;
+    this._zeroCount += previous.zeroCount;
 
-    const minScale = this._minScale(other);
+    const minScale = this._minScale(previous);
 
     this._downscale(this.scale - minScale);
 
-    this._mergeBuckets(this.positive, other, other.positive, minScale);
-    this._mergeBuckets(this.negative, other, other.negative, minScale);
+    this._mergeBuckets(this.positive, previous, previous.positive, minScale);
+    this._mergeBuckets(this.negative, previous, previous.negative, minScale);
   }
 
   /**
diff --git a/packages/sdk-metrics/test/aggregator/ExponentialHistogram.test.ts b/packages/sdk-metrics/test/aggregator/ExponentialHistogram.test.ts
index 150eb13e91..2bcbc43307 100644
--- a/packages/sdk-metrics/test/aggregator/ExponentialHistogram.test.ts
+++ b/packages/sdk-metrics/test/aggregator/ExponentialHistogram.test.ts
@@ -574,6 +574,15 @@ describe('ExponentialHistogramAggregation', () => {
       assert.deepStrictEqual(acc0.toPointValue(), acc0Snapshot);
       assert.deepStrictEqual(acc1.toPointValue(), acc1Snapshot);
     });
+
+    it("keeps the previous point's startTime", () => {
+      const agg = new ExponentialHistogramAggregator(4, true);
+      const acc0 = agg.createAccumulation([0, 0]);
+      const acc1 = agg.createAccumulation([3, 0]);
+
+      const result = agg.merge(acc0, acc1);
+      assert.strictEqual(result.startTime, acc0.startTime);
+    });
   });
 
   describe('diff', () => {