Add partition length metrics to Olric

[kwapik]

Description of change

This can be used to track number of keys and their backups in Olric.

Checklist

• [x] Tested in playground or other setup

Summary by CodeRabbit

• New Features

  • Introduced new metrics to monitor the length of cache partitions and backup partitions, enhancing visibility into cache performance and health.

• Refactor

  • Updated cache metrics collection to include new partition length measurements.

[coderabbitai[bot]]

Walkthrough

The recent changes involve enhancing the distributed cache metrics by introducing two new gauge metrics: one for the length of cache partitions and another for the length of backup cache partitions. These metrics are added to the DistCacheMetrics struct and are calculated within the DistCache type's scrapeMetrics method. Additionally, corresponding constants are defined in the metrics schema.

Changes

File(s)	Change Summary
pkg/dist-cache/dist-cache-metrics.go, pkg/dist-cache/dist-cache.go	Added PartitionsLength and BackupPartitionsLength metrics to DistCacheMetrics and updated scrapeMetrics to calculate these metrics.
pkg/metrics/schema.go	Introduced new constants for cache partition length metrics.

Poem

🐇 In the realm of cache, a change took flight,
Metrics for partitions, now in plain sight.
Lengths counted with care, in backup and main,
CodeRabbit hopped joyfully, for clarity's gain.
🌟

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