feat(monitoring): add a switch to turn off the per-request metric

[geckofu]

Problem

monitoring.WithMonitor records a request measurement for every HTTP request. When the Monitor is the log monitor (the fallback when no InfluxDB is configured), that measurement is one log line per request.

On theplant/kakuyasu-services prod-mp that is 10.3M lines a week across 15 services. On the busiest one (web) it is 42% of the whole log volume. The lines carry path, method, response code and duration, which request tracing already has.

There was no way to turn it off short of not using appkit/service.

Change

• monitoring.WithMonitorContextOnly(m): installs the Monitor in the request context but skips the request measurement. Handlers that record their own metrics through monitoring.ForceContext keep working.
• appkit/service picks between WithMonitor and WithMonitorContextOnly based on MONITORING_DISABLE_REQUEST_METRIC, loaded with configor like the other middlewares in that package, and logs the decision at startup.

Default behaviour is unchanged: without the env var set, WithMonitor runs exactly as before.

Notes

• WithMonitorContextOnly also skips the "cannot determine response code" warning, since that comes from building the measurement's tags.
• Tests cover both paths: the metric is recorded by WithMonitor, not recorded by WithMonitorContextOnly, and the Monitor lands in the request context either way.

Test runs

• go build ./...
• go test ./monitoring/... ./service/... — both pass.

🤖 Generated with Claude Code

[geckofu]

Closing: not the route we are taking. kakuyasu drops these lines at the log collector instead, matching how mcd already handles them in its firelens config. Leaving appkit's default behaviour alone.