added support for influxdb non_negative_difference function in tsdb

pkg/tsdb/influxdb/query_part_test.go

@@ -24,7 +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"))`},
+		{mode: "non_negative_difference", params: []string{}, input: "max(value)", expected: `non_negative_difference(max(value))`},
 	}
 
 	queryContext := &tsdb.TsdbQuery{TimeRange: tsdb.NewTimeRange("5m", "now")}