non_negative_difference() function in InfluxDB 1.3 (#9104)

Add support of non_negative_difference() function from InfluxDB 1.3
https://docs.influxdata.com/influxdb/v1.3/administration/differences/#functions

public/app/plugins/datasource/influxdb/query_part.ts

@@ -230,6 +230,15 @@ register({
   renderer: functionRenderer,
 });
 
+register({
+  type: 'non_negative_difference',
+  addStrategy: addTransformationStrategy,
+  category: categories.Transformations,
+  params: [],
+  defaultParams: [],
+  renderer: functionRenderer,
+});
+
 register({
   type: 'moving_average',
   addStrategy: addTransformationStrategy,