16.3The four signals I'd watch

For synchronous APIs:

Traffic
Errors
Latency
Saturation

Very similar to standard SRE thinking.

Traffic

requests/sec
records/minute

Errors

HTTP 5xx
Salesforce failures
retry exhaustion
validation failures

Latency

p50
p95
p99
downstream latency

Saturation

CPU
memory
connection pools
threads
queue backlog
Salesforce limits