Add @MetricsApi annotation

I addition to `@PublicApi`, it would be useful to annotate string literals and enum constant names which leak into emitted metrics, and thus cannot be safely renamed without compatibility bridges.

Examples: most constants in `DruidMetrics` class.

Start by reading the existing @PublicApi annotation and the constants in DruidMetrics. Determine how a new @MetricsApi annotation should identify string literals and enum constant names that are emitted as metrics, then verify that the annotated names retain compatibility when renamed.

Written by the indexing model from the issue text.