Allows custom prometheus metrics collector

[fedefernandez]

What this does?

This PR have two goals:

• Easily skip some metric collection. So now you can just send a None in some of the metric fields and that metric will be skipped.
• Easily allow to implement the metrics for some call. For example, you can customize the recordTotalTime by sending your custom totalTime Histogram and then override the DefaultPrometheusMetricsOps class.

The binary compatibility is maintained by keeping the two public methods (build and apply) untouched.

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #763 into master will decrease coverage by 0.27%.
The diff coverage is 88.46%.

@@            Coverage Diff            @@
##           master    #763      +/-   ##
=========================================
- Coverage   80.98%   80.7%   -0.28%     
=========================================
  Files          73      73              
  Lines        1162    1166       +4     
  Branches       15      15              
=========================================
  Hits          941     941              
- Misses        221     225       +4

Impacted Files	Coverage Δ
...kindness/mu/rpc/prometheus/PrometheusMetrics.scala	90% <88.46%> (-2.31%)	⬇️
...kindness/mu/rpc/srcgen/avro/AvroSrcGenerator.scala	68.25% <0%> (-4.77%)	⬇️

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