enhancement(tag_cardinality_limit): Add metric and tag name to tag_value_limit_exceeded_total metric

[kaarolch]

Summary

When using the tag_cardinality_limit transform, it's difficult to identify which specific metrics and tag keys are hitting the configured value limit. The tag_value_limit_exceeded_total metric only provides a count of exceeded events without context about which metric or tag was blocked, making it challenging to debug and monitor cardinality issues.
More in #20084

This PR adds metric_name and tag_key labels to the tag_value_limit_exceeded_total metric and allowing:

• Identify which specific metrics are hitting the limit
• Identify which tag keys are causing the limit to be exceeded
• Create more targeted alerts and dashboards

Vector configuration

sources:
  vector_metrics:
    type: "internal_metrics" # required
    scrape_interval_secs: 10 
  statsd:
    type: statsd
    address: "0.0.0.0:8128"
    mode: "udp"

transforms:
  metrics_cardinality_limit:
    type: tag_cardinality_limit
    inputs:
      - statsd
    limit_exceeded_action: drop_tag
    mode: exact
    value_limit: 10
sinks:
  drop_bh:
    type: blackhole
    inputs:
      - metrics_cardinality_limit
    print_interval_secs: 0
    buffer:
      type: memory
      when_full: drop_newest
      max_events: 100
  console_debug:
    type: console
    inputs:
      -  vector_metrics
    encoding:
      codec: json
    buffer:
       max_events: 10000
       type: memory
       when_full: drop_newest

How did you test this PR?

Build locally based on docs/DEVELOPING.md and start with config mentioned in the previous section :

target/debug/vector --config ../vector.yaml

Result:

{"name":"tag_value_limit_exceeded_total","namespace":"vector","tags":{"component_id":"metrics_cardinality_limit","component_kind":"transform","component_type":"tag_cardinality_limit","host":"xxxx","metric_name":"high_cardinality_metric_gauge_1","tag_key":"pod_name"},"timestamp":"2025-11-13T20:10:30.055509Z","kind":"absolute","counter":{"value":7470.0}}

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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  • Alternatively, we recommend running the following locally before pushing to the remote branch:
    • make fmt
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[domalessi]

A few suggestions but looks good

[kaarolch]

btw. Should we add more label? This PR introduces two new tags to an internal metric for the tag_cardinality_limit transform.

[pront]

Changes look reasonable. We could have very high cardinality here though, I wonder if we need to gate this behind a config option.