[beinan]

Production recorded /records reads taking 217–241 seconds before returning 404. The recovery endpoint retained only 5xx outcomes, while route-level latency histograms could not identify the affected table.

Retain the latest completed read taking at least five seconds in the existing bounded per-table hint cache, including its status and response-header duration. These additive schema-1 fields preserve the nonblocking recording path, five-minute expiry, 4,096-entry cap and 1 MiB response limit. Failures and slow reads precede ordinary activity when the response is truncated. Request parameters, record IDs and payloads are excluded. This supplies diagnostic hints; it does not cancel requests or infer that a slow read needs a merge.

Validation: metrics unit tests passed (10, plus one ignored manual benchmark); enabled/disabled HTTP integration tests passed, including a delayed 404, in-flight demand and payload exclusion. Tests also cover threshold boundaries, independent expiry, preservation after successful requests, lock contention and bounded truncation. cargo clippy -p lance-context-metrics --all-targets -- -D warnings, formatting and diff checks passed. This is independent of #390 and does not block its release.