Support for metrics

[rhmdnd]

The compliance service should support detailed metrics and allow SREs to configure the metrics endpoint (e.g., prometheus).

This work is imperative for failure cases we're planning to encounter. For example, the main use case for the service is to ingest compliance data from external sources (like the Compliance Operator). If a field in the compliance data fails validation at the data layer in the compliance service, it should let SREs know.

This issue is for tracking an implementation for metrics.