feat(health)!: bound OTLP sink queues

[jayzhudev]

Bound each OTLP target's independent log and metric queues so stalled exports have a defined limit. queue_capacity defaults to 32768 entries per queue (large default is used to prevent surprising drops during bursts, while still providing an upper limit). Duplicate identities retain latest-value replacement, while a new identity at capacity evicts the oldest entry.

Related issues

Resolves #5088

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Breaking Changes

• This PR contains breaking changes

The existing OTLP replacement metrics replace the target_index label with the configured endpoint in the target label. Dashboards and alerts using target_index must be updated.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

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• crates/health/src/otlp/mod.rs

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• crates/health/src/otlp/mod.rs

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Summary by CodeRabbit

• New Features

  • Added configurable per-target OTLP queue capacity, defaulting to 32,768 entries per signal.
  • Added queue depth monitoring by target and signal.
  • Added reporting for entries dropped when queues reach capacity.
  • Queues now retain the latest data while evicting the oldest entries when full.

• Bug Fixes

  • Invalid zero-capacity configurations are now rejected.

• Documentation

  • Documented OTLP queue capacity, drop behavior, restart requirements, and related metrics.

Walkthrough

OTLP targets now have configurable bounded log and metric queues. The queues evict the oldest distinct key at capacity and preserve replacements. The sinks track queue depth and emit target- and signal-labeled metrics.

Changes

OTLP queue capacity

Layer / File(s)	Summary
Bounded queue semantics crates/health/src/sink/dedup_queue.rs	DedupQueue supports capacity limits, oldest-key eviction, SaveOutcome, replacement preservation, and depth tracking. Tests cover these behaviors.
Capacity and telemetry contract crates/health/src/config.rs, crates/health/src/otlp/mod.rs, crates/health/example/config.example.toml, docs/observability/core_metrics.md	OtlpTargetConfig adds a defaulted, positive queue_capacity. OTLP drop events use endpoint and signal labels. Fixtures, parsing tests, validation tests, examples, and metric documentation were updated.
OTLP sink integration crates/health/src/sink/otlp.rs	OTLP sinks create bounded log and metric queues, process all SaveOutcome variants, expose queue depth, report evictions, and label replacement metrics by endpoint. Tests cover eviction reporting, depth, capacities, and endpoint labels.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to 4d383

The queue-bound changes are merge-ready after normal checks and review; no actionable merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant Config
  participant OtlpSink
  participant DedupQueue
  participant Metrics
  Config->>OtlpSink: provide queue_capacity and target endpoint
  OtlpSink->>DedupQueue: enqueue log or metric record
  DedupQueue-->>OtlpSink: return SaveOutcome
  OtlpSink->>Metrics: record queue depth or target-signal eviction

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: bounding OTLP sink queues.
Description check	✅ Passed	The description accurately explains bounded queues, eviction behavior, breaking changes, and testing for this changeset.
Linked Issues check	✅ Passed	The implementation addresses the linked issue objectives, including per-target capacities, eviction, replacement, metrics, and queue behavior.
Out of Scope Changes check	✅ Passed	The configuration, queue, OTLP, test, and documentation changes directly support the linked issue objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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• Create PR with unit tests

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Inline comments:
In `@crates/health/src/otlp/mod.rs`:
- Around line 62-66: In OtlpQueueEntryDropped, narrow the target and signal
fields from public visibility to crate-level visibility by using pub(crate),
while leaving the event structure and field types unchanged.

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📒 Files selected for processing (6)

• crates/health/example/config.example.toml
• crates/health/src/config.rs
• crates/health/src/otlp/mod.rs
• crates/health/src/sink/dedup_queue.rs
• crates/health/src/sink/otlp.rs
• docs/observability/core_metrics.md

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