Add options for calling elastic pipelines from events

[zpriddy]

Feature Request

Proposal:

Add in an option to set a pipeline flag based off of tags or default pipeline for the elasticsearch output plugin. This will make the elastic output plugin more flexible to custom pipeline transforms in elastic.

Current behavior:

No option to call pipelines in elastic.

Desired behavior:

Add in two new plugin inputs as shown below. ( I will be attaching a PR with this issue where the changes have been applied. )

  ## Pipeline Config
  ## To use a ingest pipeline, set this to the name of the pipeline you want to use.
  # use_pipeline = "my_pipeline"
  ## Additionally, you can specify a tag name using the notation {{tag_name}}
  ## which will be used as part of the pipeline name. If the tag does not exist,
  ## the default pipeline will be used as the pipeline. If a default pipeline is not
  ## set, then no pipeline will be used,
  # use_pipeline = "{{es_pipeline}}"
  # default_pipeline = "my_pipeline"

Use case:

This allows us to do transforms or normalization on metrics in elastic using elastic pipelines, letting us form more complex nested metrics per the elastic ECS format than telegraf natively supports.