Add gzip compression for URL metrics using Compression Streams API

[b1ink0]

Summary

Fixes #1893

Relevant technical choices

This PR implements the Compression Streams API to efficiently compress URL Metrics JSON before sending it. This significantly reduces the payload size, allowing more data to be sent without exceeding the 64 KiB limit.

Previously some of these changes where reverted due to the bug #1928.

This PR is able to avoid the issue of URL metrics not being sent on pagehide event by compressing the URL metrics before pagehide event and then recompressing it whenever URL metrics are updated by the extensions.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.51%. Comparing base (583a4e9) to head (6aa261c). Report is 27 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1959      +/-   ##
==========================================
+ Coverage   72.34%   72.51%   +0.17%     
==========================================
  Files          85       85              
  Lines        6979     7023      +44     
==========================================
+ Hits         5049     5093      +44     
  Misses       1930     1930              

Flag	Coverage Δ
multisite	72.51% <100.00%> (+0.17%)	:arrow_up:
single	40.33% <0.00%> (-0.26%)	:arrow_down:

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[b1ink0]

If we do this, we should add a pagehide event handler before we load any of the extensions, so that our pagehide callback is called first, which then causes extendElementData and extendRootData to no-op (or throw an error) because at this point we can't re-compress the URL Metric since it is too late.

https://github.com/WordPress/performance/issues/1893#issuecomment-2728201130

I would like to understand more about adding a pagehide event handler before loading any extensions. From my interpretation, the goal is to override the extendElementData and extendRootData functions to throw an error if an extension tries to modify URL metrics after the pagehide event.

However, we're currently still supporting the finalize method, even though it's deprecated. Introducing this change might cause issues for extensions that still rely on finalize.

At the moment, I’ve added a flag called enableCompression, which is set to false if a finalize function is detected. This effectively disables compression. I could use this same flag to prevent the function overrides that would otherwise throw errors.

cc: @westonruter

[westonruter]

I would like to understand more about adding a pagehide event handler before loading any extensions. From my interpretation, the goal is to override the extendElementData and extendRootData functions to throw an error if an extension tries to modify URL metrics after the pagehide event.

@b1ink0 I think that comment I made is obsolete. We should prevent compression from happening if there is a finalize function exported from an extension, like you're doing in the PR.

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

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[phanduynam]

This issue has been resolved, please close it admin