feat: Add cache size to DedupTTLCache and log cache stats

[t29-cristian]

Add cache monitoring and alerting for container metrics

Added cache size monitoring to the cluster collector to detect and alert when the container metrics cache approaches capacity, preventing silent metric data loss.

Changes:

• Added size() and utilization() methods to DedupTTLCache to report current cache usage
• Enhanced update_container_metrics endpoint with tiered logging:
  • DEBUG: Normal operations (< 80% full)
  • ERROR: High utilization warning (80-94% full)
  • CRITICAL: Cache nearly full alert (≥ 95% full)
• All log messages include cache_size, maxsize, and utilization percentage
• Alerts provide actionable guidance to increase --cache-maxsize

This makes cache exhaustion visible at default log levels (previously silent) and helps operators identify when cache capacity needs to be increased.

[github-actions]

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[t29-cristian]

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[t29-cristian]

I would argue that it is best not to have a cache limit at all in the first place by default since in kubernetes we have memory limits anyway which can take care of situations where a pod might use way more than needed. But since I am not entirely sure about the intention of the developers, I am just adding something to help me right-size for our environments and tell me when I run the risk of having values temporarily missing.

[t29-cristian]

When we would set the logger to debug now, we would see:
2026-01-16 14:57:38,669 - checkmk_kube_agent.api - DEBUG - Container metrics updated: received=1926, cache_size=18404/100000 (18.4% full)
2026-01-16 14:57:38,732 - checkmk_kube_agent.api - DEBUG - Container metrics updated: received=890, cache_size=18404/100000 (18.4% full)
[...]

The best approach would be to expose a metric that we can scrape in the cluster so that we can alert on it, but at least this way we have something to go on.