feat: Add ingester_chunks_flush_failures_total

[grobinson-grafana]

What this PR does / why we need it:

This commit adds a new metric to pkg/ingester/metrics.go to count the total number of flush failures. Loki operators should create alerts on this metric to avoid periods of recurring failures from increasing memory-pressure on ingesters due to buffering of chunks.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
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