Drop_newest discards not recorded in buffer_received or buffer_discarded metrics for non blocking disk buffers

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Problem

Events discarded from a full disk buffer configured with drop_newest do not increment buffer_received_events_total, buffer_discarded_events_total, or the corresponding byte metrics.

To reproduce bug:

1. run vector with below config
2. allow vector to run long enough for disk buffer to fill
3. curl metrics endpoint, verify that
   a. vector_component_sent_events_total{component_id="load"} > vector_buffer_received_events_total{component_id="slow"}, which indicates that events discarded from the slow buffer do not increment vector_buffer_received_events_total
   b. vector_buffer_discarded_events_total and vector_component_discarded_events_total metrics do not show up, indicating no discard metric is incremented for events dropped by the buffer. (note that only the buffer discard metric is expected to exist)

Configuration

data_dir: /tmp/vector-data

sources:
  load:
    type: demo_logs
    format: json
    interval: 0.0

  metrics:
    type: internal_metrics

sinks:
  slow:
    type: blackhole
    inputs: [load]
    rate: 1
    buffer:
      type: disk
      max_size: 268435488
      when_full: drop_newest

  prometheus:
    type: prometheus_exporter
    inputs: [metrics]
    address: 127.0.0.1:9598

Version

vector 0.58.0 (aarch64-apple-darwin 2bcad9b 2026-08-26 13:37:07.557544670)

References

Related: https://github.com/vectordotdev/vector/issues/24606, mentions that the component discard metric is not incremented in this case. This issue is for the buffer discard metric.

Fixed by: #26312

Start by reviewing the fix referenced as #26312, then reproduce the issue with the configuration in this report and query the metrics endpoint. Done means drop_newest discards are reflected in the buffer received, buffer discarded, and corresponding byte metrics without introducing unrelated component discard metrics.