app/vmselect/promql: return NaN from count() over zero time series

This aligns `count` behavior with Prometheus.

app/vmselect/promql/aggr.go

@@ -312,7 +312,11 @@ func aggrFuncCount(tss []*timeseries) []*timeseries {
 			}
 			count++
 		}
-		dst.Values[i] = float64(count)
+		v := float64(count)
+		if count == 0 {
+			v = nan
+		}
+		dst.Values[i] = v
 	}
 	return tss[:1]
 }

app/vmselect/promql/aggr_incremental.go

@@ -348,7 +348,12 @@ func mergeAggrCount(dst, src *incrementalAggrContext) {
 }
 
 func finalizeAggrCount(iac *incrementalAggrContext) {
-	// Nothing to do
+	dstValues := iac.ts.Values
+	for i, v := range dstValues {
+		if v == 0 {
+			dstValues[i] = nan
+		}
+	}
 }
 
 func updateAggrSum2(iac *incrementalAggrContext, values []float64) {

app/vmselect/promql/aggr_incremental_test.go

@@ -81,7 +81,7 @@ func TestIncrementalAggr(t *testing.T) {
 	})
 	t.Run("count", func(t *testing.T) {
 		t.Parallel()
-		valuesExpected := []float64{6, 0, 5, 5}
+		valuesExpected := []float64{6, nan, 5, 5}
 		f("count", valuesExpected)
 	})
 	t.Run("sum2", func(t *testing.T) {

app/vmselect/promql/exec_test.go

@@ -2353,10 +2353,10 @@ func TestExecSuccess(t *testing.T) {
 	})
 	t.Run(`count(multi-vector)`, func(t *testing.T) {
 		t.Parallel()
-		q := `count(label_set(10, "foo", "bar") or label_set((15-time()/100)^0.5, "baz", "sss"))`
+		q := `count(label_set(time()<1500, "foo", "bar") or label_set(time()<1800, "baz", "sss"))`
 		r := netstorage.Result{
 			MetricName: metricNameExpected,
-			Values:     []float64{2, 2, 2, 1, 1, 1},
+			Values:     []float64{2, 2, 2, 1, nan, nan},
 			Timestamps: timestampsExpected,
 		}
 		resultExpected := []netstorage.Result{r}