Latency, traffic, errors, and saturation, the four metrics Google's SRE book recommends for monitoring any service.
The four golden signals are latency (how long a request takes), traffic (how many requests per second), errors (the percent that fail), and saturation (how close the service is to its capacity limit). The framework was popularized by Google's Site Reliability Engineering book and remains the most widely-adopted minimum viable monitoring set, because together they answer the four questions a responder asks first during an incident: is it slow, is it busy, is it failing, is it about to fall over.
Most outage diagnoses start by triaging which of the four signals is degraded. A team that has all four wired up for every service can rule out 80% of root causes in under a minute. A team that has only one or two of them spends that minute scrolling logs.
See the part of the platform that handles golden signals in production.