Which Pub/Sub metrics do you find most critical for 'early warning' alerts?

[madhavmadupu]

When scaling Pub/Sub topics, there are a lot of metrics available in Cloud Monitoring. Aside from the obvious subscription/num_undelivered_messages, what are your 'must-have' alerts?

I’m particularly interested in how people monitor for 'slow consumers' vs. actual downstream outages. Are you using oldest_unacked_message_age as your primary signal, or have you found more success with custom SLIs?