Add PrometheusMetricObserver abstraction

[jotare]

Description

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How was this PR tested?

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[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #7440: Nucliadb metrics refinement.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b9c5ecd) 84.89% compared to head (6658f87) 85.20%. Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1421      +/-   ##
==========================================
+ Coverage   84.89%   85.20%   +0.31%     
==========================================
  Files         443      450       +7     
  Lines       24273    24810     +537     
==========================================
+ Hits        20606    21139     +533     
- Misses       3667     3671       +4     

Flag	Coverage Δ
ingest	74.89% <ø> (+0.21%)	:arrow_up:
nucliadb	68.24% <ø> (+0.75%)	:arrow_up:
reader	75.94% <ø> (-0.26%)	:arrow_down:
train	60.25% <ø> (ø)
utils	85.18% <ø> (+<0.01%)	:arrow_up:
writer	83.97% <ø> (-0.73%)	:arrow_down:

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see 30 files with indirect coverage changes

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