ingester: add max active series per user metric.

[jmcarp]

Max active series per user depends on default and per-tenant limits, as well as distributor sharding strategy and number of healthy ingesters. To simplify alerting when a tenant is approaching this limit, add a cortex_ingester_max_active_series metric that is updated when the existing cortex_ingester_active_series changes.

Note: sorry if I'm missing something, but I didn't see an existing metric that I could use to alert on tenants approaching the max active series limit in a straightforward way. Let me know if this makes sense!

Signed-off-by: Josh Carp [email protected]

What this PR does:

Which issue(s) this PR fixes: Fixes #

Checklist

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

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