fix: Server Timing - Remove reliance on performance API

[patrickhousley]

Removing reliance on the browser resources API for the calculation of New Relic server time. On site with many resource requests, there was a small chance that the RUM network call would be flushed from the resources API cache before we could capture it and calculate server time.

Overview

Simplifying the NR server time calculation to use our own captured start and end times for the RUM network request. On our internal NR site, we were rarely seeing an error related to the time keeper not being able to calculate server time. The cause was found to be the RUM call missing from the resources API buffer. This is most likely because there is a max buffer size in the browser after which the browser starts dropping older resource requests.

We decided to not use a more precise method of setting up a resource observer simply because it has a lot of code and logic overhead for little gain (hundreds of milliseconds). We are also planning to cache the server time across page loads and tabs.

Related Issue(s)

N/A - Pre-release issue

Testing

[github-actions]

Asset Size Report

Merging this pull request will result in the following asset size changes:

Agent	Asset	Previous Size	New Size	Diff
lite	loader	32.28 kB / 11.27 kB (gzip)	31.98 kB / 11.17 kB (gzip)	-0.94% / -0.87% (gzip)
lite	async-chunk	46.67 kB / 15.33 kB (gzip)	46.72 kB / 15.35 kB (gzip)	0.11% / 0.09% (gzip)
pro	loader	51.45 kB / 17.48 kB (gzip)	51.14 kB / 17.41 kB (gzip)	-0.59% / -0.45% (gzip)
pro	async-chunk	88.05 kB / 27.04 kB (gzip)	88.1 kB / 27.05 kB (gzip)	0.06% / 0.04% (gzip)
spa	loader	60.01 kB / 20.11 kB (gzip)	59.71 kB / 20 kB (gzip)	-0.5% / -0.55% (gzip)
spa	async-chunk	102.26 kB / 31.2 kB (gzip)	102.32 kB / 31.22 kB (gzip)	0.05% / 0.06% (gzip)
lite-polyfills	loader	127.5 kB / 40.98 kB (gzip)	127.21 kB / 40.89 kB (gzip)	-0.23% / -0.22% (gzip)
lite-polyfills	async-chunk	60.19 kB / 17.58 kB (gzip)	60.23 kB / 17.58 kB (gzip)	0.06% / 0.03% (gzip)
pro-polyfills	loader	147.53 kB / 46.91 kB (gzip)	147.24 kB / 46.81 kB (gzip)	-0.2% / -0.2% (gzip)
pro-polyfills	async-chunk	122.51 kB / 31.95 kB (gzip)	122.55 kB / 31.96 kB (gzip)	0.03% / 0.03% (gzip)
spa-polyfills	loader	155.55 kB / 49.02 kB (gzip)	155.26 kB / 48.93 kB (gzip)	-0.19% / -0.19% (gzip)
spa-polyfills	async-chunk	139.01 kB / 36.43 kB (gzip)	139.04 kB / 36.44 kB (gzip)	0.03% / 0.03% (gzip)

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 80.49%. Comparing base (7a1ba34) to head (fa19ce2).

❗ Current head fa19ce2 differs from pull request most recent head 2083047. Consider uploading reports for the commit 2083047 to get more accurate results

Files	Patch %	Lines
src/features/page_view_event/aggregate/index.js	0.00%	4 Missing ⚠️
src/common/timing/time-keeper.js	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #930      +/-   ##
==========================================
- Coverage   80.56%   80.49%   -0.07%     
==========================================
  Files         154      154              
  Lines        7058     7050       -8     
  Branches     1398     1395       -3     
==========================================
- Hits         5686     5675      -11     
- Misses       1168     1171       +3     
  Partials      204      204              

Flag	Coverage Δ
unit-tests	58.29% <33.33%> (-0.13%)	⬇️

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