Auto-generate metric emitter mappings

[ztzxt]

Description

Whenever a new metric is added or existing ones are changed, metric mapping files in emitters should be automatically updated.

Example files:

https://github.com/apache/druid/blob/master/extensions-contrib/prometheus-emitter/src/main/resources/defaultMetrics.json
https://github.com/apache/druid/blob/master/extensions-contrib/statsd-emitter/src/main/resources/defaultMetricDimensions.json
https://github.com/apache/druid/blob/master/extensions-contrib/graphite-emitter/src/main/resources/defaultWhiteListMap.json

Motivation

As soon as a new metric is introduced, it has to be defined manually for each emitter to be available. This creates a maintenance load and state difference between emitters. Since all of the mapping files has some pre-defined schema, updating these files should be automated.

[cryptoe]

@ztzxt Do you want to take this up ?. We can always guide you. The logic would be a little brittle since we would have to parse out the metric from metrics.md file. Instead what we can do, as part of the release process, we can make a python script which parses out this logic from the metrics file and creates the relevant files. This can be done by the release manager one time before the release.

[ztzxt]

Is metrics.md updated manually or is it generated from code? If it is generated from code, we can implement this in where the doc is generated. Otherwise, accepting the metrics.md as a source of truth makes sense. Also, could you point me to it if there is already a process for running scripts pre-release?