Option to send all non-numeric fields as tags

[passing]

The option "use_event_fields_for_data_points" is very helpful, just for our use cases we always need only numeric fields to be data points, but all string fields to be tags.

With an option like "use_string_fields_for_tags", you would not need to manually list all fields from different documents that should to be sent to Influxdb as tags.