New Issue/PR tags: Compatibility and Metrics

[Roman Leventov]

I've added two tags for issues and PRs:

https://github.com/druid-io/druid/labels/Compatibility: opposite to "Incompatible", for PRs that restore compatibility, broken earlier.

https://github.com/druid-io/druid/labels/Metrics: for issues and PRs about changing emitting metrics, and/or changing QueryMetrics interface or any of it's query type specific subinterfaces. Since https://github.com/druid-io/druid/pull/3954 there is QueryMetrics interface, which is supposed to be broken often, and implemented in extensions. People who implement it in extensions may want to track breaking changes and prevent unwanted changes, or influence the design on PR review stage, before changes are merged into Druid source and released.