Avoid redundant labelset protobuf copy

[jotare]

Description

We can ParseFromString directly without an intermediate copy

How was this PR tested?

Existing test suite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.48%. Comparing base (267d243) to head (b35a960).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3841      +/-   ##
==========================================
- Coverage   85.50%   85.48%   -0.03%     
==========================================
  Files         573      573              
  Lines       50913    50911       -2     
  Branches    15695    15695              
==========================================
- Hits        43532    43520      -12     
- Misses       6743     6754      +11     
+ Partials      638      637       -1     

Flag	Coverage Δ
nidx	81.29% <ø> (+<0.01%)	⬆️
nucliadb	74.48% <100.00%> (-0.07%)	⬇️
nucliadb-ingest	43.86% <0.00%> (+<0.01%)	⬆️
nucliadb-reader	43.85% <0.00%> (+<0.01%)	⬆️
nucliadb-search	54.82% <0.00%> (+<0.01%)	⬆️
nucliadb-standalone	46.18% <100.00%> (-0.03%)	⬇️
nucliadb-train	44.82% <100.00%> (-0.01%)	⬇️
nucliadb-writer	47.36% <100.00%> (+0.03%)	⬆️
nucliadb_dataset	73.42% <ø> (ø)
nucliadb_models	69.72% <ø> (ø)
nucliadb_sdk	82.30% <ø> (ø)
nucliadb_telemetry	85.67% <ø> (ø)
nucliadb_utils	80.54% <ø> (ø)

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