Add Starlark parsing example of nested JSON (influxdata#8675)

* Add Starlark parsing example of nested JSON

* add correct Example Input and Output fields to make script testable

* removed logging from script

* remove logging.star load statement

plugins/processors/starlark/testdata/json_nested.star

@@ -0,0 +1,45 @@
+# 
+# This code assumes the value parser with data_type='string' is used
+# in the input collecting the JSON data. The entire JSON obj/doc will
+# be set to a Field named `value` with which this code will work.
+
+# JSON:
+#  ```
+#     {
+#         "fields": {
+#             "LogEndOffset": 339238,
+#             "LogStartOffset": 339238,
+#             "NumLogSegments": 1,
+#             "Size": 0,
+#             "UnderReplicatedPartitions": 0
+#         },
+#         "name": "partition",
+#         "tags": {
+#             "host": "CUD1-001559",
+#             "jolokia_agent_url": "http://localhost:7777/jolokia",
+#             "partition": "1",
+#             "topic": "qa-kafka-connect-logs"
+#         },
+#         "timestamp": 1591124461
+#     } ```
+# 
+# Example Input:
+# json value="[{\"fields\": {\"LogEndOffset\": 339238, \"LogStartOffset\": 339238, \"NumLogSegments\": 1, \"Size\": 0, \"UnderReplicatedPartitions\": 0}, \"name\": \"partition\", \"tags\": {\"host\": \"CUD1-001559\", \"jolokia_agent_url\": \"http://localhost:7777/jolokia\", \"partition\": \"1\", \"topic\": \"qa-kafka-connect-logs\"}, \"timestamp\": 1591124461}]"
+
+# Example Output:
+# partition,host=CUD1-001559,jolokia_agent_url=http://localhost:7777/jolokia,partition=1,topic=qa-kafka-connect-logs LogEndOffset=339238i,LogStartOffset=339238i,NumLogSegments=1i,Size=0i,UnderReplicatedPartitions=0i 1610056029037925000
+
+
+load("json.star", "json")
+
+def apply(metric):
+  j_list = json.decode(metric.fields.get('value')) # input JSON may be an arrow of objects
+  metrics = []
+  for obj in j_list:
+    new_metric = Metric("partition") # We want a new InfluxDB/Telegraf metric each iteration
+    for tag in obj["tags"].items(): # 4 Tags to iterate through
+      new_metric.tags[str(tag[0])] = tag[1]
+    for field in obj["fields"].items(): # 5 Fields to iterate through
+      new_metric.fields[str(field[0])] = field[1]
+    metrics.append(new_metric)
+  return metrics