Add prometheus metrics

[erikjohnston]

This now makes prometheus dependency mandatory. It adds two metrics:

1. Received request rate by servlet
2. Number of emails sent

[erikjohnston]

Mypy is failing on sydent/sydent.py:200: error: Call to untyped function (unknown) in typed context [no-untyped-call], which is:

sydent/sydent/sydent.py

Line 200 in 24fe02d

prometheus_client.start_http_server(

I'm not entirely sure what to do about this.

[DMRobertson]

Mypy is failing on sydent/sydent.py:200: error: Call to untyped function (unknown) in typed context [no-untyped-call], which is:

sydent/sydent/sydent.py

Line 200 in 24fe02d

prometheus_client.start_http_server(

I'm not entirely sure what to do about this.

This is on me: I made sydent enforce mypy --strict/ Options:

• # type: ignore
• change mypy config to allow untyped calls
• write stubs for prometheus_client by hand
• try installing a stub package
• try bumping the package to see if it includes type hints

Sounds like the last option sufficed

[erikjohnston]

Thank you!