Implement opt-in PERFLAB_PLACE_OBJECT_CACHE_DROPIN for Server-Timing object-cache.php placement

[b1ink0]

Summary

Fixes #1633

Relevant technical choices

Introduce PERFLAB_PLACE_OBJECT_CACHE_DROPIN constant to opt-in to placing the Server-Timing object-cache.php drop-in.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.50%. Comparing base (ff8b62b) to head (922212d).
Report is 131 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/performance-lab/load.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1996      +/-   ##
==========================================
- Coverage   72.51%   72.50%   -0.02%     
==========================================
  Files          85       88       +3     
  Lines        7023     7146     +123     
==========================================
+ Hits         5093     5181      +88     
- Misses       1930     1965      +35     

Flag	Coverage Δ
multisite	72.50% <50.00%> (-0.02%)	⬇️
single	40.14% <50.00%> (-0.20%)	⬇️

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[github-actions]

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Co-authored-by: b1ink0 <[email protected]>
Co-authored-by: felixarntz <[email protected]>
Co-authored-by: mukeshpanchal27 <[email protected]>
Co-authored-by: westonruter <[email protected]>

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[felixarntz]

@b1ink0 Thanks for the PR! Basically looks good to go, just a few nit-picks that would be great to address before merge.

[felixarntz]

Nit-pick: Maybe move this below, either right before or right after the PERFLAB_DISABLE_SERVER_TIMING clause? Since it's more closely related to that check.

It would also be great if you could update the comment above PERFLAB_DISABLE_OBJECT_CACHE_DROPIN, to clarify that it's maintained only for backward compatibility.

[b1ink0]

Done in f492eed

[felixarntz]

Thanks @b1ink0!