Prometheus Metrics Exporter for Path Resolution

[makr-code]

Context

This issue implements the roadmap item 'Prometheus Metrics Exporter for Path Resolution' for the config domain. It is sourced from the consolidated roadmap under 🟠 High Priority — Near-term (v1.5.0 – v1.8.0) and targets milestone v1.7.0.

Primary detail section: Prometheus Metrics Exporter for Path Resolution

Goal

Deliver the scoped changes for Prometheus Metrics Exporter for Path Resolution in src/config/ and complete the linked detail section in a release-ready state for v1.7.0.

Detailed Scope

Prometheus Metrics Exporter for Path Resolution

Priority: High
Target Version: v1.7.0

ConfigPathResolver::Metrics tracks resolution_hits, resolution_misses, legacy_fallbacks, cache_hits, cache_misses, and unmapped_requests via std::atomic<uint64_t>. Expose these as Prometheus gauges/counters so operations teams can detect when legacy paths are still in use and monitor migration progress.

Implementation Notes:

• [x] Create config_metrics_exporter.cpp; register with the server's Prometheus registry at startup (prometheus/registry.h).
• [x] Metric names: themis_config_resolution_hits_total, themis_config_resolution_misses_total, themis_config_legacy_fallbacks_total, themis_config_cache_hit_ratio (derived), themis_config_unmapped_requests_total.
• [x] Add label category to themis_config_legacy_fallbacks_total using ConfigPathResolver::inferCategory() (already private method) to show which config category has the most legacy usage.
• [x] Export function called every scrape interval (pull model); read from ConfigPathResolver::metrics() atomics — no mutex needed.
• [x] Add themis_config_cache_capacity and themis_config_cache_ttl_seconds info metrics for dashboard context.

Performance Targets:

• Metrics scrape completes in < 1 ms (atomic reads, no cache iteration).
• Zero impact on ConfigPathResolver::resolve() hot path (metrics are already incremented by existing atomic ops).

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Acceptance Criteria

• Create config_metrics_exporter.cpp; register with the server's Prometheus registry at startup (prometheus/registry.h).
• Metric names: themis_config_resolution_hits_total, themis_config_resolution_misses_total, themis_config_legacy_fallbacks_total, themis_config_cache_hit_ratio (derived), themis_config_unmapped_requests_total.
• Add label category to themis_config_legacy_fallbacks_total using ConfigPathResolver::inferCategory() (already private method) to show which config category has the most legacy usage.
• Export function called every scrape interval (pull model); read from ConfigPathResolver::metrics() atomics — no mutex needed.
• Add themis_config_cache_capacity and themis_config_cache_ttl_seconds info metrics for dashboard context.
• Metrics scrape completes in < 1 ms (atomic reads, no cache iteration).
• Zero impact on ConfigPathResolver::resolve() hot path (metrics are already incremented by existing atomic ops).

Relationships

• Roadmap row: Update documentation to reflect v1.0.0 implementation state #57 (🟠 High Priority — Near-term (v1.5.0 – v1.8.0))
• Depends on: none identified during generation.
• Part of: consolidated roadmap delivery tracking.

References

• src/ROADMAP.md
• src/config/FUTURE_ENHANCEMENTS.md#prometheus-metrics-exporter-for-path-resolution
• Source key: roadmap:57:config:v1.7.0:prometheus-metrics-exporter-for-path-resolution

Generated from the consolidated source roadmap. Keep the roadmap and issue in sync when scope changes.