Switch pkg/ingester to promauto.With(reg)

[pracucci]

What this PR does:
Following the work related to #2204 to get rid of global metrics and registerer, in this PR I've migrated pkg/ingester metrics to promauto.With(registerer).

Few notes:

• There's still a global metric in pkg/ingester/client which I will address in a separate PR
• Not much happy with the path followed for the WAL and memorySeries. The problem is that the initial design doesn't look much clean in regards of global variables and I had to deal with that in order to keep the changeset small (but I admit there's much room for improvement design-wise there)

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[codesome]

Not much familiar with the integration code yet, but other parts LGTM

[pstibrany]

Looks good to me, thanks for doing this cleanup.

[gouthamve]

Not much happy with the path followed for the WAL and memorySeries.

Can you clarify this? Just curious as I didn't find anything obviously off when I glanced through the code. Is this about the integration tests or something about the WAL metrics themselves?

[pracucci]

Can you clarify this? Just curious as I didn't find anything obviously off when I glanced through the code. Is this about the integration tests or something about the WAL metrics themselves?

@gouthamve I haven't been clear, sorry. My comment is about the change I did to pass the metric instances to some functions (batchSend(), newMemorySeries(), processCheckpointRecord()) because it looks a bad design to me. I'm not happy with my changes but I picked that path to avoid to do a larger refactoring and keeping the change set easier to diff and understand.