InfluxDB: Flux: make $__interval and $__interval_ms work in alerting (#38889)

* influxdb: flux: handle $__interval and $__interval_ms in alert-queries

* influxdb: flux: do not handle interval-variable in the frontend

* $__interval should be rounded

* added comment

pkg/tsdb/influxdb/flux/executor.go

@@ -18,11 +18,7 @@ const maxPointsEnforceFactor float64 = 10
 func executeQuery(ctx context.Context, query queryModel, runner queryRunner, maxSeries int) (dr backend.DataResponse) {
 	dr = backend.DataResponse{}
 
-	flux, err := interpolate(query)
-	if err != nil {
-		dr.Error = err
-		return
-	}
+	flux := interpolate(query)
 
 	glog.Debug("Executing Flux query", "flux", flux)