Revert "app/vmselect/promql: remove metric name after applying ceil, floor and round functions in order to be more consistent with Prometheus"

This reverts commit ac45082216.

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 names:

* ceil
* floor
* round

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

app/vmselect/promql/transform.go

@@ -18,8 +18,11 @@ import (
 )
 
 var transformFuncsKeepMetricGroup = map[string]bool{
+	"ceil":      true,
 	"clamp_max": true,
 	"clamp_min": true,
+	"floor":     true,
+	"round":     true,
 }
 
 var transformFuncs = map[string]transformFunc{