Revert "app/vmselect/promql: remove metric name after applying clamp_min and clamp_max functions in order to be consistent with Prometheus"

This reverts commit bb61a4769b.

Reason for revert: the previous behavior for VictoriaMetrics is easier to understand and use by users -
functions, which don't change the meaning of the time series shouldn't drop metric name.

Now the following functions do not drop metric name:

* clamp_min
* clamp_max

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/674

app/vmselect/promql/exec_test.go

@@ -636,6 +636,7 @@ func TestExecSuccess(t *testing.T) {
 			Values:     []float64{1000, 1200, 1400, 1400, 1400, 1400},
 			Timestamps: timestampsExpected,
 		}
+		r.MetricName.MetricGroup = []byte("foobar")
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
@@ -647,6 +648,7 @@ func TestExecSuccess(t *testing.T) {
 			Values:     []float64{1000, 1200, 1400, 1400, 1400, 1400},
 			Timestamps: timestampsExpected,
 		}
+		r.MetricName.MetricGroup = []byte("foobar")
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})

app/vmselect/promql/transform.go

@@ -17,6 +17,11 @@ import (
 	"github.com/valyala/histogram"
 )
 
+var transformFuncsKeepMetricGroup = map[string]bool{
+	"clamp_max": true,
+	"clamp_min": true,
+}
+
 var transformFuncs = map[string]transformFunc{
 	// Standard promql funcs
 	// See funcs accepting instant-vector on https://prometheus.io/docs/prometheus/latest/querying/functions/ .
@@ -125,8 +130,12 @@ func newTransformFuncOneArg(tf func(v float64) float64) transformFunc {
 }
 
 func doTransformValues(arg []*timeseries, tf func(values []float64), fe *metricsql.FuncExpr) ([]*timeseries, error) {
+	name := strings.ToLower(fe.Name)
+	keepMetricGroup := transformFuncsKeepMetricGroup[name]
 	for _, ts := range arg {
-		ts.MetricName.ResetMetricGroup()
+		if !keepMetricGroup {
+			ts.MetricName.ResetMetricGroup()
+		}
 		tf(ts.Values)
 	}
 	return arg, nil