Auto-generate metric emitter mappings

### 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.

Start by comparing the schemas and contents of extensions-contrib/prometheus-emitter/src/main/resources/defaultMetrics.json, statsd-emitter/src/main/resources/defaultMetricDimensions.json, and graphite-emitter/src/main/resources/defaultWhiteListMap.json. Determine how metric additions and changes can update these mapping files automatically; done means the emitter mappings stay synchronized without manual definitions for each emitter.

Written by the indexing model from the issue text.