fix: extract searcher metric from experiment payload

[ashtonG]

Ticket

ET-655

Description

This fixes an issue where the searcher metric wasn't decoded from the experiment api payload.

Test Plan

on the searches table, add the "searcher metric" column to the view. It should be populated.

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 49.80%. Comparing base (20ed126) to head (1bd1786).
Report is 11 commits behind head on main.

Files	Patch %	Lines
webui/react/src/services/decoder.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9768      +/-   ##
==========================================
- Coverage   54.12%   49.80%   -4.33%     
==========================================
  Files        1257      934     -323     
  Lines      154820   125488   -29332     
  Branches     3486     3485       -1     
==========================================
- Hits        83804    62502   -21302     
+ Misses      70870    62840    -8030     
  Partials      146      146              

Flag	Coverage Δ
harness	?
web	53.40% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
webui/react/src/services/decoder.ts	21.57% <0.00%> (-0.03%)	⬇️

... and 323 files with indirect coverage changes

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[keita-determined]

tested locally, and confired searcherMetric has values