feat(sample transform): add optional discarded_event_tags callback

[p-parekh]

Summary

Adds an optional with_discarded_event_tags builder on the Sample transform's runtime type. When set, the callback is invoked at the drop site and its returned (key, value) pairs are merged into the component_discarded_events_total counter labels. Default None — behavior unchanged for every existing caller.

Motivation

Downstream wrappers (concretely, the Observability Pipelines Worker — a Datadog-internal consumer that uses Vector as a library) configure Sample with a group_by template but cannot currently attach per-event group-by tags to the discard metric. The discard counter today carries only intentional:true, so when a customer pipeline drops events the operator has to grep logs to know which group caused the drops.

A wrapping fork or duplicating Sample wholesale was rejected; this small extension point lets library consumers attach the tags they own without Vector taking a position on what the tags mean.

Vector configuration

No new TOML/YAML config. The callback is a programmatic API used by library consumers, not the Vector binary. Existing pipelines using sample see no behavior change.

How did you test this PR?

• Added discarded_event_tags_callback_is_invoked_on_drop in src/transforms/sample/tests.rs that:
  • constructs a Sample with an aggressive rate=100
  • attaches a callback that increments an atomic counter and returns a fixed tag pair
  • drives 200 random events through Sample::transform
  • asserts the callback was invoked at least 100 times (most events drop)
• Confirmed existing tests pass unchanged (the discarded_event_tags field is None in every existing test path).
• Manually verified the existing SampleEventDiscarded emission path is taken when the callback is None, so the ComponentEventsDropped<INTENTIONAL> plumbing is unaffected for current users.

Change Type

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

Is this a breaking change?

• Yes
• No

The new field defaults to None. Sample::new and Sample::new_with_dynamic signatures are unchanged. The only addition is the with_discarded_event_tags builder method.

Does this PR include user facing changes?

• Yes. Added changelog.d/<id>_sample_discarded_event_tags.enhancement.md.

Library consumers gain a new opt-in API; pipeline operators see no change unless a downstream consumer of Vector wires the callback through.

Notes

• The metric continues to be component_discarded_events_total; only the label set varies based on the (optional) callback's return value.
• When the callback is set, the drop site emits the counter directly (via metrics::counter!) and logs a debug message in the same shape as ComponentEventsDropped<INTENTIONAL>::emit would. This is necessary because ComponentEventsDropped registers its counter with a fixed label set at registration time, so dynamic per-event labels cannot flow through it.
• The bypass is only active when the callback is Some. With None, the existing emit!(SampleEventDiscarded) path runs unchanged.
• The callback signature takes &Event (not &LogEvent) so trace-event drops also benefit if a future consumer wants per-trace tags. Today the existing trace-event handling in Sample::transform calls emit!(SampleEventDiscarded) via the same branch, so it gains tag support automatically when the callback is set.

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