Seeing isn't enough. You also have to decide how much failure is acceptable, and the time to decide is before anything fails.
SLI and SLO
Also called Service level indicator, Service level objective, SLA
An SLI is a measure of how your service is doing, like the share of requests that succeed in time. An SLO is the target you set for that measure, like 99.9% over 30 days.
Without an agreed number, "is it reliable enough?" is an argument about feelings, and perfect isn't on offer. With one, you know when to keep building and when to stop and fix things. Choose what counts as good with care, because that choice can hide a real problem.
“Propose one SLI and one SLO for checkout: the share of POST /checkout requests that return under 500 ms and aren't 5xx, measured at the load balancer, over 30 days. Compute the last 30 days from real logs and tell me where we are before you suggest a target.”