Rule group metrics are always zero in Cortex

[bboreham]

#1548 prompted me to look at these metrics. Most of what I looked at was zero:

prometheus_rule_group_duration_seconds (this one is also zero in our Prometheus)
prometheus_rule_group_duration_seconds_count
prometheus_rule_group_duration_seconds_sum
prometheus_rule_group_last_duration_seconds
prometheus_rule_group_last_evaluation_timestamp_seconds

In fact the only one which isn't all zeros in Cortex is prometheus_rule_group_rules

[jtlisi]

The metrics that should work with the way we utilize the rule group struct are

prometheus_rule_evaluation_duration_seconds
prometheus_rule_evaluation_failures_total
prometheus_rule_group_rules

I think the ones you listed could be added relatively easily with some upstream work

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[bboreham]

Emitting a bunch of metrics which are all zero strikes me as wasteful, though this is by no means the only place that Cortex does it.

[jtlisi]

Ever since #1571, all of the rule group metrics in cortex have been populated. I think this issue can be closed. I agree there is room for improvement on null metrics generally.

[bboreham]

Nit-picking, the exact metrics named are no longer exported at all by Cortex, but prefixed ones like cortex_prometheus_rule_group_duration_seconds are now exported per-user. Thanks!