fix(incremental_to_absolute transform): fix LRU cache memory handling and emit cache metrics

[GreyLilac09]

Summary

• Create metrics for cache size, events, and evictions
• Always track cache size (for metrics tracking)
• Previously, the size for the event values for gauges (and other metrics) was always 0, which is inaccurate and messes up memory tracking
• Get the item after inserting to LRU cache, ensuring it moves to the back of the LRU cache and doesn't get evicted immediately if the cache is full
• Added some tests for incremental_to_absolute, absolute_to_incremental, and LRU cache capacity policy behavior

Vector configuration

data_dir: ./vector-data-dir
sources:
  s0:
    type: statsd
    address: "0.0.0.0:8127"
    mode: udp
  internal_metrics:
    scrape_interval_secs: 10
    type: internal_metrics
transforms:
  t0:
    type: incremental_to_absolute
    inputs:
      - s0
    cache:
      max_bytes: 5000
      max_events: 100
  filter_cache_metrics:
    type: filter
    inputs:
      - internal_metrics
    condition: '.name == "component_cache_evictions_total" || .name == "component_cache_bytes" || .name == "component_cache_events"'
sinks:
  blackhole:
    type: blackhole
    inputs:
      # - s0
      - t0
  file:
    type: file
    inputs:
      - filter_cache_metrics
    path: /vector-%Y-%m-%d.log
    encoding:
      codec: json

How did you test this PR?

Graph showing new metrics:

Graph showing consistent allocated bytes for incremental_to_absolute transform when max_bytes is set:

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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