prom: add support for default step param (#9866)

Alerting for prometheus have been depending on the step parameter from each query.
In https://github.com/grafana/grafana/pull/9226 we changed the behavior for step in the
frontend which caused problems for alerting. This commit fixes that by introducing a default
min interval value so alerting always have something to depend on. 

closes #9777

pkg/tsdb/influxdb/model_parser.go

@@ -2,9 +2,11 @@ package influxdb
 
 import (
 	"strconv"
+	"time"
 
 	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/tsdb"
 )
 
 type InfluxdbQueryParser struct{}
@@ -37,13 +39,7 @@ func (qp *InfluxdbQueryParser) Parse(model *simplejson.Json, dsInfo *models.Data
 		return nil, err
 	}
 
-	interval := model.Get("interval").MustString("")
-	if interval == "" && dsInfo.JsonData != nil {
-		dsInterval := dsInfo.JsonData.Get("timeInterval").MustString("")
-		if dsInterval != "" {
-			interval = dsInterval
-		}
-	}
+	parsedInterval, err := tsdb.GetIntervalFrom(dsInfo, model, time.Millisecond*1)
 
 	return &Query{
 		Measurement:  measurement,
@@ -53,7 +49,7 @@ func (qp *InfluxdbQueryParser) Parse(model *simplejson.Json, dsInfo *models.Data
 		Tags:         tags,
 		Selects:      selects,
 		RawQuery:     rawQuery,
-		Interval:     interval,
+		Interval:     parsedInterval,
 		Alias:        alias,
 		UseRawQuery:  useRawQuery,
 	}, nil