Starlark script for renaming prometheus remote write metrics (#9074)

plugins/parsers/prometheusremotewrite/README.md

@@ -16,9 +16,7 @@ Converts prometheus remote write samples directly into Telegraf metrics. It can
   data_format = "prometheusremotewrite"
 ```
 
-### Example
-
-**Example Input**
+### Example Input
 ```
 prompb.WriteRequest{
 		Timeseries: []*prompb.TimeSeries{
@@ -38,7 +36,11 @@ prompb.WriteRequest{
 
 ```
 
-**Example Output**
+### Example Output
 ```
 prometheus_remote_write,instance=localhost:9090,job=prometheus,quantile=0.99 go_gc_duration_seconds=4.63 1614889298859000000
 ```
+
+**For alignment with the [InfluxDB v1.x Prometheus Remote Write Spec](https://docs.influxdata.com/influxdb/v1.8/supported_protocols/prometheus/#how-prometheus-metrics-are-parsed-in-influxdb)**
+
+- Use the [Starlark processor rename prometheus remote write script](https://github.com/influxdata/telegraf/blob/master/plugins/processors/starlark/testdata/rename_prometheus_remote_write.star) to rename the measurement name to the fieldname and rename the fieldname to value. 

plugins/processors/starlark/README.md

@@ -237,6 +237,7 @@ def apply(metric):
 - [multiple metrics from json array](/plugins/processors/starlark/testdata/multiple_metrics_with_json.star) - Builds a new metric from each element of a json array then returns all the created metrics.
 - [custom error](/plugins/processors/starlark/testdata/fail.star) - Return a custom error with [fail](https://docs.bazel.build/versions/master/skylark/lib/globals.html#fail).
 - [compare with previous metric](/plugins/processors/starlark/testdata/compare_metrics.star) - Compare the current metric with the previous one using the shared state.
+- [rename prometheus remote write](/plugins/processors/starlark/testdata/rename_prometheus_remote_write.star) - Rename prometheus remote write measurement name with fieldname and rename fieldname to value.
 
 [All examples](/plugins/processors/starlark/testdata) are in the testdata folder.
 

plugins/processors/starlark/testdata/rename_prometheus_remote_write.star

@@ -0,0 +1,16 @@
+# Specifically for prometheus remote write - renames the measurement name to the fieldname. Renames the fieldname to value. 
+# Assumes there is only one field as is the case for prometheus remote write. 
+#
+# Example Input:
+# prometheus_remote_write,instance=localhost:9090,job=prometheus,quantile=0.99 go_gc_duration_seconds=4.63 1614889298859000000
+#
+# Example Output:
+# go_gc_duration_seconds,instance=localhost:9090,job=prometheus,quantile=0.99 value=4.63 1614889299000000000
+
+def apply(metric):
+   if metric.name == "prometheus_remote_write":
+        for k, v in metric.fields.items():
+            metric.name = k
+            metric.fields["value"] = v
+            metric.fields.pop(k)
+   return metric