[log_to_metrics] add_label adds a dimension just like label_field, producing duplicate metrics

[pbraun9]

how to reproduce

[SERVICE]
  flush              1
  log_level          info

[INPUT]
  Name               dummy
  Dummy              {"block_device":"vda", "used_percent":"0.01"}
  Tag                log.io

[INPUT]
  Name               dummy
  Dummy              {"block_device":"vdb", "used_percent":"0.02"}
  Tag                log.io

[FILTER]
  name               log_to_metrics
  match              log.io
  tag                metric.io
  metric_mode        gauge
  metric_name        used_percent
  value_field        used_percent
  metric_description disk i/o
  add_label          block_device block_device

[OUTPUT]
  name stdout
  match log.io

[OUTPUT]
  name stdout
  match metric.io

result is

[0] log.io: [[1784185288.876221477, {}], {"block_device"=>"vda", "used_percent"=>"0.01"}]
[0] log.io: [[1784185288.876303412, {}], {"block_device"=>"vdb", "used_percent"=>"0.02"}]
2026-07-16T07:01:28.876267678Z log_metric_gauge_used_percent{block_device="vda"} = 0.01
2026-07-16T07:01:27.876286303Z log_metric_gauge_used_percent{block_device="vdb"} = 0.02
2026-07-16T07:01:28.876267678Z log_metric_gauge_used_percent{block_device="vda"} = 0.01
2026-07-16T07:01:28.876310684Z log_metric_gauge_used_percent{block_device="vdb"} = 0.02

seems like a bug to me. or is that normal?