chore(internal metrics): move config_reload_* metrics to VectorReload*

[pront]

Summary

Unifies reload metrics by removing duplicate config_reload_rejected and config_reloaded metrics in favor of existing component_errors_total and reloaded_total metrics. This significantly reduces complexity and simplifies monitoring.

• Now component_errors_total with error_code="reload" include a reason tag with values:

  • global_options_changed - Non-reloadable global options changed
  • global_diff_failed - Failed to compute global config diff
  • topology_build_failed - New topology failed to build/healthcheck
  • restore_failed - Rollback failed (fatal)

• Refactored reload_config_and_respawn

  • return type from Result<(bool, Option<ReloadRejectionReason>), ()> to a much cleaner Result<(), ReloadError>
  • this allows for handling all emit! logic in a single place

We can consider this a non-breaking change - the removed metrics were never actually emitted due to abug, and adding the reason tag is a non-breaking change. Happy we caught this early.

Vector configuration

#data_dir: /Users/pavlos.rontidis/CLionProjects/vector/pront/data/data_dir_0

api:
  enabled: true

sources:
  s0:
    type: internal_metrics
    scrape_interval_secs: 10

transforms:
  filter_config_metrics:
    type: filter
    inputs: ["s0"]
    condition:
      type: vrl
      source: |
        starts_with!(.name, "reload")

sinks:
  console:
    inputs: ["filter_config_metrics"]
    type: console
    encoding:
      codec: "json"
#      pretty: true

How did you test this PR?

Run:

cargo run -- -w --config /Users/pavlos.rontidis/CLionProjects/vector/pront/configs/reload_metrics.yaml

Edited the fields multiple times and observed the internal metrics.

Also, updated existing unit tests.

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

relates: #24150

Notes

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[datadog-vectordotdev]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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