Embed metrics into structs and optional initialization

[jtlisi]

The single binary version of cortex ends up reporting metrics for each component in cortex regardless of whether the module is enabled or not. At this point it would make sense to start embedding metrics into structs so that the initialization of those metrics is actually dependent on whether that module is deployed or not.

This can be done piecemeal one module or package at a time.

[bboreham]

I support this idea; came here to add that we have some other metrics that come in via promauto from vendor'd code that never gets called. Example prometheus_remote_storage_highest_timestamp_in_seconds

[jtlisi]

Relevant Design Document: https://docs.google.com/document/d/100HSvgvaswVR96Bmz2BWIL3d9ds6wqNTVI7_vESgOtE/edit?usp=sharing

[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.

[pracucci]

I think the solution we've already started adopting is to pass prometheus.Registerer to services and register metrics on that, avoiding promauto.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[bboreham]

This issue says not to use promauto, while #2204 says to use promauto.

This one got closed as stale; does that mean promauto won?

[pracucci]

This one got closed as stale; does that mean promauto won?

I think we can close this issue. The pattern we're currently using (and looks working fine with downstream projects) is to take the prometheus.Registerer in input (eg. NewSomething(reg prometheus.Registerer) and then registering metrics via promauto.With(reg).

[bboreham]

Agreed: if we create the metrics in NewSomething() that achieves the aim of this issue.

Nitpick: the key is we are creating metrics via promauto.With(), hence not needing to think about whether they need registering.