Add more Prometheus metrics

[foxcpp]

• [ ] endpoint: Counter of panics in IMAP/SMTP endpoints, should they happen
• [ ] remote: Negotiated TLS version
• [ ] endpoint: Negotiated TLS version
• [ ] remote: Amount of sent messages (?)
• [ ] ???

[foxcpp]

Gonna add "help wanted" label in the hope somebody will add more ideas on what should be exposed as metrics.

[foxcpp]

Also I believe it could be useful to turn existing runtime/trace instrumentation into histograms, but there are many regions defined, that may turn it into a lot of histograms. Perhaps we should turn only "tasks" into them. That anyway will need some abstraction since runtime/trace API does not expose any such statistics.

[Allineer]

@foxcpp

maddy_storage_messages{mailbox=""}
maddy_target_received_total{module="", name=""}
maddy_target_sent_total{module="", name=""}
maddy_target_rejected_total{module="", name="", smtp_code="", smtp_enchcode=""}
maddy_endpoint_received_total{module="", address=""}
maddy_endpoint_sent_total{module="", address=""}
maddy_endpoint_rejected_total{module="", address="", smtp_code="", smtp_enchcode=""}