Expose instance AZ in ring via Prometheus metric

[roystchiang]

What this PR does:

Add a new metric cortex_ring_availability_zone for displaying which availability zone an instance is configured to run in. This helps with monitoring via Prometheus, and can help detect if there's an unbalance in AZ distribution.

Which issue(s) this PR fixes: Fixes #

Checklist

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

[stale[bot]]

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