metrics: Enable font and glyph cache memory metrics in Cobalt

[Awallky]

This change enables the reporting of font-related memory usage metrics to UMA, which were previously defined in the engine but not collected.

• Initialize the memory instrumentation client in the Cobalt browser process to enable process-level allocator dumps.
• Update CobaltMetricsServiceClient to explicitly extract and record Skia (sk_glyph_cache) and Blink (font_caches/shape_caches) allocator dumps.
• Add new font cache metrics to the memory histograms allowlist.
• Add a new browsertest (font_metrics_browsertest) to verify that the new histograms are correctly populated on a running device.
• Add documentation for the identified font metrics in cobalt/docs/.

These metrics provide visibility into the memory footprint of character rasterization and text shaping caches, aiding in identifying font-related memory regressions and optimizing resource usage.

Bug: 483758673

[github-actions]

🤖 Gemini Suggested Commit Message

---

cobalt: Enable font and glyph cache memory metrics

Initialize the memory instrumentation client in the browser process
to enable process-level allocator dumps. Update the metrics service
client to explicitly extract and record Skia (sk_glyph_cache) and
Blink (font_caches/shape_caches) allocator dumps. Add new font
cache metrics to the memory histograms allowlist and a browsertest
to verify their population. Also, add documentation for the new
metrics.

These changes provide visibility into the memory footprint of
character rasterization and text shaping caches, aiding in
identifying font-related memory regressions and optimizing
resource usage.

Bug: 483758673

---

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[gemini-code-assist]

Code Review

This pull request successfully enables font and glyph cache memory metrics, providing valuable insights into memory usage. The changes are well-structured, including the necessary initialization, metric collection logic, a new browser test, and documentation.

I've provided a few suggestions to improve the precision of the new metrics, enhance test coverage, and ensure the documentation is consistent with the implementation. Overall, this is a solid contribution.

[sherryzy]

Overall looks good.