bugfix sort after merging histogram metrics

CHANGELOG.md

@@ -6,6 +6,7 @@
 * [ENHANCEMENT] Tenant Federation: Add a local cache to regex resolver. #7363
 * [ENHANCEMENT] Query Scheduler: Add `cortex_query_scheduler_tracked_requests` metric to track the current number of requests held by the scheduler. #7355
 * [BUGFIX] Fix nil when ingester_query_max_attempts > 1. #7369
+* [BUGFIX] Metrics Helper: Fix non-deterministic bucket order in merged histograms by sorting buckets after map iteration, matching Prometheus client library behavior. #7379
 
 ## 1.21.0 in progress
 

pkg/util/metrics_helper.go

@@ -1129,6 +1129,10 @@ func mergeHistogram(mf1, mf2 *dto.Metric) {
 			ccValue := cumulativeCount
 			newBucket = append(newBucket, &dto.Bucket{UpperBound: &ubValue, CumulativeCount: &ccValue})
 		}
+		// Sort buckets by UpperBound to ensure deterministic order
+		sort.Slice(newBucket, func(i, j int) bool {
+			return newBucket[i].GetUpperBound() < newBucket[j].GetUpperBound()
+		})
 		h1.Bucket = newBucket
 	}
 }