Dimension Mapping Source

[vjkoskela]

We already have a MappingSource which allows a single metric name to be modified and even cloned into other metric name(s). This solved lots of problems with unmodifiable metric sources (e.g. CollectD) allowing metric namespaces to be normalized (e.g. cpu samples across cores; slash instead of period; etc.). However, as we've moved more to dimensional data sources (e.g. Dog Statsd and Telegraf) it is no longer sufficient to simply modify the metric name.

There are use cases where we may need to:

1. Modify dimensions (keys and/or values)
2. Render dimensions into the metric name
3. Extract dimensions from the metric name

The extraction functionality is being prototyped in the new GraphitePlaintextToRecordParser; however, we may want to consider generalizing it and supporting it outside any specific parser.