app/vmselect/promql: add label_value(q, label_name) func, which returns numeric value labels with name label_name in q

app/vmselect/promql/exec_test.go

@@ -1312,6 +1312,44 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`label_value()`, func(t *testing.T) {
+		t.Parallel()
+		q := `with (
+			x = (
+				label_set(time(), "foo", "123.456", "__name__", "aaa"),
+				label_set(-time(), "foo", "bar", "__name__", "bbb"),
+				label_set(-time(), "__name__", "bxs"),
+				label_set(-time(), "foo", "45", "bar", "xs"),
+			)
+		)
+		sort(x + label_value(x, "foo"))`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{-955, -1155, -1355, -1555, -1755, -1955},
+			Timestamps: timestampsExpected,
+		}
+		r1.MetricName.Tags = []storage.Tag{
+			{
+				Key:   []byte("bar"),
+				Value: []byte("xs"),
+			},
+			{
+				Key:   []byte("foo"),
+				Value: []byte("45"),
+			},
+		}
+		r2 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1123.456, 1323.456, 1523.456, 1723.456, 1923.456, 2123.456},
+			Timestamps: timestampsExpected,
+		}
+		r2.MetricName.Tags = []storage.Tag{{
+			Key:   []byte("foo"),
+			Value: []byte("123.456"),
+		}}
+		resultExpected := []netstorage.Result{r1, r2}
+		f(q, resultExpected)
+	})
 	t.Run(`label_transform(mismatch)`, func(t *testing.T) {
 		t.Parallel()
 		q := `label_transform(time(), "__name__", "foobar", "xx")`

app/vmselect/promql/transform.go

@@ -63,6 +63,7 @@ var transformFuncs = map[string]transformFunc{
 	"label_copy":         transformLabelCopy,
 	"label_move":         transformLabelMove,
 	"label_transform":    transformLabelTransform,
+	"label_value":        transformLabelValue,
 	"union":              transformUnion,
 	"":                   transformUnion, // empty func is a synonim to union
 	"keep_last_value":    transformKeepLastValue,
@@ -881,6 +882,31 @@ func labelReplace(tss []*timeseries, srcLabel string, r *regexp.Regexp, dstLabel
 	return tss, nil
 }
 
+func transformLabelValue(tfa *transformFuncArg) ([]*timeseries, error) {
+	args := tfa.args
+	if err := expectTransformArgsNum(args, 2); err != nil {
+		return nil, err
+	}
+	labelName, err := getString(args[1], 1)
+	if err != nil {
+		return nil, fmt.Errorf("cannot get label name: %s", err)
+	}
+	rvs := args[0]
+	for _, ts := range rvs {
+		ts.MetricName.ResetMetricGroup()
+		labelValue := ts.MetricName.GetTagValue(labelName)
+		v, err := strconv.ParseFloat(string(labelValue), 64)
+		if err != nil {
+			v = nan
+		}
+		values := ts.Values
+		for i := range values {
+			values[i] = v
+		}
+	}
+	return rvs, nil
+}
+
 func transformLn(v float64) float64 {
 	return math.Log(v)
 }