Refactor metrics

[KalmanMeth]

Description

Refactor Metrics code so that it is common for Prometheus and OpenTelemetry metrics.
This removes duplicate code and now gives us the implementation for OpenTelemetry histograms.

Dependencies

n/a

Checklist

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• Will this change affect NetObserv / Network Observability operator? If not, you can ignore the rest of this checklist.
• Is this PR backed with a JIRA ticket? If so, make sure it is written as a title prefix (in general, PRs affecting the NetObserv/Network Observability product should be backed with a JIRA ticket - especially if they bring user facing changes).
• Does this PR require product documentation?
  • If so, make sure the JIRA epic is labelled with "documentation" and provides a description relevant for doc writers, such as use cases or scenarios. Any required step to activate or configure the feature should be documented there, such as new CRD knobs.
• Does this PR require a product release notes entry?
  • If so, fill in "Release Note Text" in the JIRA.
• Is there anything else the QE team should know before testing? E.g: configuration changes, environment setup, etc.
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• QE requirements (check 1 from the list):
  • Standard QE validation, with pre-merge tests unless stated otherwise.
  • Regression tests only (e.g. refactoring with no user-facing change).
  • No QE (e.g. trivial change with high reviewer's confidence, or per agreement with the QE team).

[codecov]

Codecov Report

Attention: Patch coverage is 68.00000% with 80 lines in your changes are missing coverage. Please review.

Project coverage is 67.06%. Comparing base (5e89c89) to head (9b8e42d).
Report is 11 commits behind head on main.

Files	Patch %	Lines
...ipeline/encode/opentelemetry/encode_otlpmetrics.go	23.63%	42 Missing ⚠️
pkg/pipeline/encode/metrics_common.go	79.59%	22 Missing and 8 partials ⚠️
pkg/pipeline/encode/encode_prom.go	83.33%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #590      +/-   ##
==========================================
+ Coverage   66.04%   67.06%   +1.02%     
==========================================
  Files         103      104       +1     
  Lines        7509     7405     -104     
==========================================
+ Hits         4959     4966       +7     
+ Misses       2256     2143     -113     
- Partials      294      296       +2     

Flag	Coverage Δ
unittests	67.06% <68.00%> (+1.02%)	⬆️

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[jpinsonneau]

Small comments, otherwise looks good ! Thanks @KalmanMeth

[jpinsonneau]

any reason to keep this outside of metrics_common ?

[jpinsonneau]

Maybe move this file under a new prometheus folder ?

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[jotak]

/lgtm