The history is process-wide, and tests cleared it only after running, and
not all of them. Records from earlier tests or an earlier -count repetition
reached later assertions: TestRecordUpstreamOperations left a record for
TestRecentFailuresNewestFirst, and the server tests never cleared it.
- Export ResetRecentFailures so tests outside the metrics package can use it
- Clear the history before and after every test that records or reads
failures directly, in metrics, server and handler
- Assert exactly one failure in the handler tests instead of looking only
at the newest
- Add ui_failure_details / PROXY_UI_FAILURE_DETAILS, off by default
- Each failure row expands to its latest failures, newest first
- Show a note naming the config key when failures exist and the flag is off
- Document the flag in README, configuration reference and example config
