Propagate current spans in multi-shard search subtasks

[jotare]

Description

Describe the proposed changes made in this PR.

How was this PR tested?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.42%. Comparing base (c129671) to head (7576bc8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3779      +/-   ##
==========================================
+ Coverage   84.58%   85.42%   +0.84%     
==========================================
  Files         571      571              
  Lines       50426    50431       +5     
  Branches    15291    15296       +5     
==========================================
+ Hits        42653    43082     +429     
+ Misses       7144     6720     -424     
  Partials      629      629              

Flag	Coverage Δ
nidx	81.02% <100.00%> (+0.09%)	⬆️
nucliadb	73.90% <ø> (+1.98%)	⬆️
nucliadb-ingest	43.87% <ø> (+0.01%)	⬆️
nucliadb-reader	43.69% <ø> (ø)
nucliadb-search	54.84% <ø> (ø)
nucliadb-standalone	46.03% <ø> (ø)
nucliadb-train	44.68% <ø> (ø)
nucliadb-writer	47.22% <ø> (-0.08%)	⬇️
nucliadb_dataset	73.40% <ø> (ø)
nucliadb_models	69.72% <ø> (ø)
nucliadb_sdk	82.28% <ø> (ø)
nucliadb_telemetry	85.67% <ø> (ø)
nucliadb_utils	80.87% <ø> (ø)

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