Basic prometheus /metrics added

[nghduc97]

Description:

#143

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• 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.

[instantdreams]

What would the endpoint be for this? http://[container-ip]:[container-port]/metrics?

I'd very much like to see this implemented.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.74%. Comparing base (ecc8655) to head (b77719d).
Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   99.37%   99.74%   +0.37%     
==========================================
  Files           7       11       +4     
  Lines         794     1161     +367     
==========================================
+ Hits          789     1158     +369     
+ Misses          5        3       -2     

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[caronc]

Made some minor changes and merged it. Sorry to take so long to review this. The expressed interest brought your efforts back to life.

Great work and thanks for the PR! 🙏 🚀