add basic prometheus metrics

[nghduc97]

Description:

https://github.com/caronc/apprise-api/issues/143

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [ ] Tests added

[caronc]

Just curious what is being monitored. Apprise-API has no backend database to collect statistics to be shared through Prometheus.

[nghduc97]

It can be used to monitor uptime, HTTP traffic/errors like when 400s or 500s status code.

Some other metrics like python GC stats come with the default too, though I don't need it for a program this size.