[Docs]: docs/metrics.md embeds 3 plots from a dead endpoint (amp-project-metrics.appspot.com returns 404 sitewide) #40558

Description

docs/metrics.md embeds three plots from the AMP project metrics service, and the whole metrics endpoint is gone: every URL on amp-project-metrics.appspot.com now returns HTTP 404, so the doc renders with three broken images.

Expected: the three charts (absolute coverage, cherry-pick issue count, release granularity) render from the metrics service. Current: all three image references are dead, and the service root itself is 404 as well — the App Engine app appears to have been shut down rather than any single path having rotted.

Reproduction Steps

1. Open https://github.com/ampproject/amphtml/blob/main/docs/metrics.md — the three embedded plots do not render.
2. Fetch the referenced URLs (browser User-Agent, redirects followed), all HTTP 404:
   • https://amp-project-metrics.appspot.com/api/plot/AbsoluteCoverageMetric.png
   • https://amp-project-metrics.appspot.com/api/plot/CherrypickIssueCountMetric.png
   • https://amp-project-metrics.appspot.com/api/plot/ReleaseGranularityMetric.png
3. Control checks on the same host, same conditions — also 404, confirming an endpoint-wide outage rather than three individual files:
   • https://amp-project-metrics.appspot.com/api/plot/TotalBuildTimeMetric.png → 404
   • https://amp-project-metrics.appspot.com/api/plot/AccessibilityViolationsMetric.png → 404
   • https://amp-project-metrics.appspot.com/ (service root) → 404

Relevant Logs

2026-10-08 ~05:55 UTC, browser UA, redirects followed:
404  https://amp-project-metrics.appspot.com/api/plot/AbsoluteCoverageMetric.png
404  https://amp-project-metrics.appspot.com/api/plot/CherrypickIssueCountMetric.png
404  https://amp-project-metrics.appspot.com/api/plot/ReleaseGranularityMetric.png
404  https://amp-project-metrics.appspot.com/api/plot/TotalBuildTimeMetric.png   (control)
404  https://amp-project-metrics.appspot.com/api/plot/AccessibilityViolationsMetric.png (control)
404  https://amp-project-metrics.appspot.com/                                    (control)
200  https://github.com/ampproject/amphtml/blob/main/docs/metrics.md             (doc itself is live)

Notes

Either the metrics service needs to be restored/republished, or docs/metrics.md should be updated/retired so it doesn't ship three permanently broken charts.

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