refactor: remove legacy NATS metrics and stats_handler

[keivenchang]

Overview:

This PR removes the legacy NATS metrics reporting infrastructure from the runtime, eliminating ~1200 lines of unused code while maintaining all passing tests.

Details:

• Removed NATS metric constants and prometheus metric structs (ComponentNatsServerPrometheusMetrics, DRTNatsClientPrometheusMetrics)
• Removed nats_metrics_worker background task that collected NATS service metrics
• Removed stats_handler infrastructure including EndpointStatsHandler type and component/service.rs file
• Removed start_stats_service() and add_stats_service() methods from DistributedRuntime
• Updated runtime examples to remove stats_handler usage
• Updated tests to remove NATS metrics filtering
• Clarified distributed_runtime.md documentation

Where should the reviewer start?

• lib/runtime/src/metrics.rs - Main metrics removal (~573 lines)
• lib/runtime/src/distributed.rs - Removed stats service methods
• lib/runtime/src/component/service.rs - Deleted file

Related Issues:

/coderabbit profile chill

[grahamking]

Can we also remove start_stats_service and add_stats_service from this file, and all of lib/runtime/src/component/service.rs ?

[keivenchang]

Yes I'll remove these unused code with pleasure. Coming up!

[grahamking]

Is it Xmas already?!

[coderabbitai]

Walkthrough

The PR removes Prometheus metrics instrumentation for NATS throughout the runtime codebase. This includes deleting NATS-specific metric definitions, metrics wrapper structs, metrics integration in initialization paths, test helpers, and adjusting test assertions to account for fewer metrics.

Changes

Cohort / File(s)	Summary
NATS Metrics Infrastructure lib/runtime/src/metrics/prometheus_names.rs, lib/runtime/src/metrics.rs	Removed public modules nats_client and nats_service, and public constants DRT_NATS_METRICS and COMPONENT_NATS_METRICS. Updated metrics.rs to remove NATS-related imports and test helpers like remove_nats_lines and extract_nats_lines.
NATS Metrics Wrapper Structs lib/runtime/src/transports/nats.rs, lib/runtime/src/service.rs	Removed public struct DRTNatsClientPrometheusMetrics from nats.rs and ComponentNatsServerPrometheusMetrics from service.rs, along with their associated impl blocks and methods.
Runtime Initialization lib/runtime/src/distributed.rs, lib/runtime/src/component.rs	Removed NATS metrics initialization in DistributedRuntime setup (metrics creation, callback registration, and background task spawning). Removed ComponentNatsServerPrometheusMetrics import from component.rs.
Test Updates lib/runtime/src/metrics.rs, lib/runtime/src/system_status_server.rs, tests/utils/payloads.py	Removed NATS metrics filtering logic in test assertions, disabled NATS-related integration tests in metrics.rs, and updated minimum dynamo_component_\* metric requirement from 23 to 11 in payloads.py.
Documentation docs/design_docs/distributed_runtime.md	Updated Component initialization description to clarify NATS service group naming using {namespace_name}.{service_name} format and its registration in the internal registry.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

• lib/runtime/src/distributed.rs: Large block of metrics initialization code removed; verify all cleanup is complete and no dangling references remain.
• lib/runtime/src/metrics.rs: Significant test suite modifications and removal of NATS-related integration tests; ensure test logic remains sound after filtering removal.
• lib/runtime/src/metrics/prometheus_names.rs: Public API surface reduction with module and constant removals; verify no external dependents rely on these exports.
• tests/utils/payloads.py: Metric threshold change from 23 to 11; confirm this aligns with the actual metric reduction and doesn't mask test failures.

Poem

🐰 Out with NATS metrics tangled tight,
Simpler signals, cleaner sight!
Runtime hops with lighter grace,
Prometheus clutter finds its place. 🌟

Pre-merge checks

✅ Passed checks (3 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title accurately describes the main objective of the changeset: removing legacy NATS metrics and stats_handler infrastructure.
Description check	✅ Passed	The description follows the template structure with Overview, Details, Where should the reviewer start, and Related Issues sections, providing comprehensive information about the changes.

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