promql: Fix counter reset detection in subqueries with histograms

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

promql/engine.go

@@ -1548,6 +1548,28 @@ func (ev *evaluator) evalSubquery(ctx context.Context, subq *parser.SubqueryExpr
 		VectorSelector: vs,
 	}
 	for _, s := range mat {
+		// Set any "NotCounterReset" and "CounterReset" hints in native
+		// histograms to "UnknownCounterReset" because we might
+		// otherwise miss a counter reset happening in samples not
+		// returned by the subquery, or we might over-detect counter
+		// resets if the sample with a counter reset is returned
+		// multiple times by a high-res subquery. This intentionally
+		// does not attempt to be clever (like detecting if we are
+		// really missing underlying samples or returning underlying
+		// samples multiple times) because subqueries on counters are
+		// inherently problematic WRT counter reset handling, so we
+		// cannot really solve the problem for good. We only want to
+		// avoid problems that happen due to the explicitly set counter
+		// reset hints and go back to the behavior we already know from
+		// float samples.
+		for i, hp := range s.Histograms {
+			switch hp.H.CounterResetHint {
+			case histogram.NotCounterReset, histogram.CounterReset:
+				h := *hp.H // Shallow copy is sufficient, we only change CounterResetHint.
+				h.CounterResetHint = histogram.UnknownCounterReset
+				s.Histograms[i].H = &h
+			}
+		}
 		vs.Series = append(vs.Series, NewStorageSeries(s))
 	}
 	return ms, mat.TotalSamples(), ws