Monitoring setup

Two failure modes, and most teams manage both simultaneously: nothing is monitored, so problems surface as customer complaints; and everything is alerted, so the channel is noise and real alerts are missed.

The target is a small number of signals that mean something, and an even smaller number of alerts that always warrant waking someone.

1. Instrument the four signals that matter

For any request-serving system, start here and resist adding more until these are solid: