A goal metric tells you whether a change helped. A guardrail tells you whether it hurt something you care about more.
Good guardrails are
- Fast: they move within minutes, such as error rate or latency
- Hard to argue with: nobody wants more failed payments
- Few: two or three, or every rollout will pause on noise
Start here
Error rate and p95 latency for the service the change touches. Add a business guardrail, such as checkout completion, once you trust the first two.