Emit a metric when pghoard quits main loop

[Aiqin-Aiven]

for example: for alerting purpose

About this change - What it does

Emit a metric when there is unrecoverable error to quit main loop So we can do some alert on services.

Why this way

A metric will generate each time if there is a main loop quit.

[Aiqin-Aiven]

will need to address rohmu PR first: https://github.com/Aiven-Open/rohmu/pull/144

[codecov-commenter]

Codecov Report

Merging #598 (dee324f) into main (6e6c70b) will decrease coverage by 0.09%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #598      +/-   ##
==========================================
- Coverage   91.44%   91.35%   -0.09%     
==========================================
  Files          32       32              
  Lines        4709     4710       +1     
==========================================
- Hits         4306     4303       -3     
- Misses        403      407       +4     

Files	Coverage Δ
pghoard/pghoard.py	85.48% <0.00%> (-0.14%)	:arrow_down:

... and 1 file with indirect coverage changes