feat: Prometheus /metrics data exportation

[42atomys]

Is your feature request related to a problem? Please describe.
No monitoring is actually available on Webhooked. To provide Webhooked production ready, we need a monitoring endpoint

Describe the solution you'd like
Actually, no data, logs or metrics is exposed about processing of webhook entries

Describe alternatives you've considered
I think create a Prometheus endpoints /metrics with prometheus go package to expose basic informations like req_counts, req_status, req_execution_time, webhook_entrypoint, webhook_kind, webhook_version, has_secure, storage_counts, storage_process_time, storage_kind, etc...