Add metrics for config-load failures and startup latency (#303)

[leynos]

Summary

Closes #303

Adds the metrics instrumentation requested as a follow-up to PR #297: a config-load failure counter and a startup-latency histogram, plus developer documentation.

Changes

• Cargo.toml: add the metrics façade (runtime) and metrics-util (dev, debugging feature).
• src/main.rs: introduce resolve_configuration (spans cli::resolve_merged_diag_json through cli::merge_with_config) and record_config_load_metrics, emitting:
  • netsuke_config_load_total — counter labelled outcome (success/failure);
  • netsuke_config_load_duration_seconds — duration histogram.
    The merge error path is extracted into handle_config_load_error. Because metrics is a façade, the instruments are no-ops until an operator installs a recorder; Netsuke bundles none.
• docs/developers-guide.md: new Configuration-load observability subsection documenting counter names, label conventions, and suggested histogram buckets.

Testing

• Unit tests use metrics_util::debugging::DebuggingRecorder + metrics::with_local_recorder to assert the counter carries outcome=failure/outcome=success and that the histogram records exactly one sample.

Structured log fields (operation, error_category) and per-phase counter labels are the scope of the follow-up #304.

Validation

• make check-fmt / make markdownlint / make lint / make test — pass (37 suites)

🤖 Generated with Claude Code

Summary by Sourcery

Instrument startup configuration loading with metrics and document their usage.

New Features:

• Add metrics-based instrumentation for configuration-load outcomes and durations during startup.

Enhancements:

• Refactor configuration resolution into a dedicated function and centralised error handler to support metrics collection.

Build:

• Add metrics and metrics-util crates to support runtime instrumentation and test-time inspection of metrics.

Documentation:

• Document configuration-load observability, including metric names, semantics, and naming conventions in the developer guide.

Tests:

• Add tests using a debugging metrics recorder to verify emitted configuration-load counters, labels, and histograms.

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✨ Finishing Touches

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• Create PR with unit tests
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[sourcery-ai]

Reviewer's Guide

Adds config-load observability by instrumenting startup configuration resolution with metrics, refactoring error handling, and documenting the new metrics, along with tests using a debugging recorder.

Sequence diagram for configuration-load metrics and error handling

sequenceDiagram
    participant main
    participant resolve_configuration
    participant cli as cli_merge
    participant metrics_facade
    participant handle_config_load_error

    main->>resolve_configuration: resolve_configuration(parsed_cli, matches)
    resolve_configuration->>cli: cli::resolve_merged_diag_json(parsed_cli, matches)
    resolve_configuration-->>resolve_configuration: DiagMode::from_json_enabled(...)
    resolve_configuration->>cli: cli::merge_with_config(parsed_cli, matches)
    resolve_configuration-->>metrics_facade: record_config_load_metrics(elapsed, merged.is_ok())
    metrics_facade-->>metrics_facade: metrics::histogram!(CONFIG_LOAD_DURATION_SECONDS)
    metrics_facade-->>metrics_facade: metrics::counter!(CONFIG_LOAD_TOTAL)
    resolve_configuration-->>main: (mode, merged)

    alt [merge succeeded]
        main-->>main: merged.with_default_command()
        main-->>main: configure_runtime(...)
    else [merge failed]
        main->>handle_config_load_error: handle_config_load_error(err, mode)
        handle_config_load_error-->>main: ExitCode::FAILURE
    end

Loading

File-Level Changes

Change	Details	Files
Instrument configuration-load phase with metrics and refactor startup configuration resolution and error handling.	Introduce resolve_configuration to compute diagnostic mode, perform config merge, and time the combined config-load phase. Add CONFIG_LOAD_TOTAL counter and CONFIG_LOAD_DURATION_SECONDS histogram, and implement record_config_load_metrics to emit them via the metrics facade. Extract handle_config_load_error to centralize config-load failure rendering and exit-code mapping, reusing prior JSON vs human-path behavior. Update run_with_args to use resolve_configuration and handle_config_load_error, calling with_default_command only on successful merges. Add unit tests validating counter labeling for success/failure and that exactly one histogram sample is recorded per invocation.	src/main.rs
Document configuration-load observability and metric conventions for operators and developers.	Add a Configuration-load observability subsection describing where instrumentation lives and how it behaves with the metrics facade. Document the two emitted instruments, their semantics, and suggested histogram bucket boundaries. Clarify metric naming and label cardinality conventions to guide future metrics additions.	docs/developers-guide.md
Wire in metrics dependencies for runtime use and test-time debugging.	Add the metrics crate as a runtime dependency for metrics facade macros. Add metrics-util with debugging feature as a dev-dependency to support DebuggingRecorder-based tests. Update Cargo.lock to capture the new dependency graph.	Cargo.toml Cargo.lock

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#303	Instrument the config-load error handling paths (including handle_config_load_error / resolve_diag_mode_or_exit / merge_cli_or_exit equivalents) with a counter labelled by outcome (success/failure) to track configuration-load failure rates.	✅
#303	Wrap the startup configuration-resolution phase, from cli::resolve_merged_diag_json through cli::merge_with_config, in a duration histogram to record startup latency.	✅
#303	Add developer documentation to docs/developers-guide.md describing the configuration observability instrumentation, including counter names, label conventions, and histogram buckets.	✅

Possibly linked issues

• Add metrics instrumentation for config-load failure rates and startup latency #303: PR implements the issue’s requested config-load failure counter, startup latency histogram, and developer documentation exactly.

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