added support for influxdb non_negative_difference function in tsdb for alerting

pkg/tsdb/influxdb/query_part.go

@@ -33,6 +33,7 @@ func init() {
 	renders["sum"] = QueryDefinition{Renderer: functionRenderer}
 	renders["mode"] = QueryDefinition{Renderer: functionRenderer}
 	renders["cumulative_sum"] = QueryDefinition{Renderer: functionRenderer}
+	renders["non_negative_difference"] = QueryDefinition{Renderer: functionRenderer}
 
 	renders["holt_winters"] = QueryDefinition{
 		Renderer: functionRenderer,

pkg/tsdb/influxdb/query_part_test.go

@@ -24,6 +24,7 @@ func TestInfluxdbQueryPart(t *testing.T) {
 		{mode: "count", params: []string{}, input: "distinct(value)", expected: `count(distinct(value))`},
 		{mode: "mode", params: []string{}, input: "value", expected: `mode(value)`},
 		{mode: "cumulative_sum", params: []string{}, input: "mean(value)", expected: `cumulative_sum(mean(value))`},
+		{mode: "non_negative_difference", params: []string{}, input: "max("value")", expected: `non_negative_difference(max("value"))`},
 	}
 
 	queryContext := &tsdb.TsdbQuery{TimeRange: tsdb.NewTimeRange("5m", "now")}