Move PrometheusMetric class from EE to CE

Summary

Contributes to #42640 (closed)

Move PrometheusMetric and PrometheusMetricEnums from EE domain to CE.

Improvements

Moving the generic alerts endpoint feature from EE to CE requires a lot of code changes. All the required changes can be split and moved in several steps (as described here).

Risks

Involved components

Optional: Intended side effects

Optional: Missing test coverage