Provide an option to enable the registry debug endpoint.

[cmfcruz]

The debug endpoint can be used for monitoring the registry metrics, and for profiling the server.

The following configuration block has to be added under the http options in the registry configuration to enable metrics:

http:
  debug:
    addr:  <some_ip_or_host>:<some_port>
    prometheus:
      enabled: true
      path: /metrics