app/vmselect/promql: properly handle Prometheus staleness marks in re…

…moveCounterResets functions

Prometheus stalenss marks shouldn't be changed in removeCounterResets. Otherwise they will be converted to an ordinary NaN values,
which couldn't be removed in dropStaleNaNs() function later. This may result in incorrect calculations for rollup functions.

Updates #1526

app/vmselect/promql/rollup.go

@@ -704,8 +704,6 @@ func getMaxPrevInterval(scrapeInterval int64) int64 {
 }
 
 func removeCounterResets(values []float64) {
-	// There is no need in handling NaNs here, since they are impossible
-	// on values from vmstorage.
 	if len(values) == 0 {
 		return
 	}
@@ -723,7 +721,9 @@ func removeCounterResets(values []float64) {
 			}
 		}
 		prevValue = v
-		values[i] = v + correction
+		if !decimal.IsStaleNaN(v) {
+			values[i] = v + correction
+		}
 	}
 }