promql: Fix HistogramStatsIterator usage for subqueries

Signed-off-by: beorn7 <beorn@grafana.com>

promql/engine.go

@@ -3891,20 +3891,34 @@ func detectHistogramStatsDecoding(expr parser.Expr) {
 			return nil
 		}
 
+	pathLoop:
 		for i := len(path) - 1; i >= 0; i-- { // Walk backwards up the path.
+			if _, ok := path[i].(*parser.SubqueryExpr); ok {
+				// If we ever see a subquery in the path, we
+				// will not skip the buckets. We need the
+				// buckets for correct counter reset detection.
+				n.SkipHistogramBuckets = false
+				break pathLoop
+			}
 			call, ok := path[i].(*parser.Call)
 			if !ok {
-				continue
+				continue pathLoop
 			}
 			switch call.Func.Name {
 			case "histogram_count", "histogram_sum", "histogram_avg":
+				// We allow skipping buckets preliminarily. But
+				// we will continue through the path to see if
+				// we find a subquery (or a histogram function)
+				// further up (the latter wouldn't make sense,
+				// but no harm in detecting it).
 				n.SkipHistogramBuckets = true
 			case "histogram_quantile", "histogram_fraction":
+				// If we ever see a function that needs the
+				// whole histogram, we will not skip the
+				// buckets.
 				n.SkipHistogramBuckets = false
-			default:
-				continue
+				break pathLoop
 			}
-			break
 		}
 		return errors.New("stop")
 	})