Feature request: Wait time of the oldest scheduled job as a metric

[mdcollins05]

When creating an internal SLO for Buildkite agents, the ScheduledJobsCount is good for knowing how many scheduled jobs you have at once but doesn't give you an idea of how long a scheduled job is waiting before being picked up.

This makes it hard to determine if we are having capacity issues and if jobs are waiting an excessive amount of time before being picked up by an agent, or if the queue is temporarily busy.

Cheers

[kj4ezj]

This would be useful to our team, as well. This is the metric we alert on.