PGCatalog: Metrics & Fix paging

[javitonino]

Fix PGCatalog paging: stop using merge_results and work directly with the models. This is also a bit faster, we skip a query per resource (to fetch labels, we get them from the catalog directly).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (1b0c11c) to head (d78fb16).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2347      +/-   ##
==========================================
- Coverage   86.48%   86.39%   -0.10%     
==========================================
  Files         369      369              
  Lines       22555    22560       +5     
==========================================
- Hits        19507    19491      -16     
- Misses       3048     3069      +21     

Flag	Coverage Δ
nucliadb	72.92% <100.00%> (-0.19%)	⬇️
nucliadb-ingest	29.53% <26.66%> (+0.01%)	⬆️
nucliadb-reader	25.16% <26.66%> (+0.01%)	⬆️
nucliadb-search	37.42% <66.66%> (+0.02%)	⬆️
nucliadb-standalone	47.81% <66.66%> (+0.02%)	⬆️
nucliadb-train	46.74% <66.66%> (+0.02%)	⬆️
nucliadb-writer	40.49% <26.66%> (+0.01%)	⬆️
nucliadb_dataset	60.09% <ø> (ø)
nucliadb_models	85.33% <ø> (ø)
nucliadb_sdk	77.48% <ø> (ø)
nucliadb_sidecar	94.23% <ø> (ø)
nucliadb_telemetry	86.57% <ø> (ø)
nucliadb_utils	82.81% <ø> (ø)

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