Wrap up metrics library

design.md

@@ -505,12 +505,14 @@ ParseResult parse_commit_request(const char* json, CommitRequest& out);
 **Test Executables:**
 - `test_arena_allocator` - Arena allocator functionality tests
 - `test_commit_request` - JSON parsing and validation tests
+- `test_metric` - Metrics system functionality tests
 - Main server executable (compiled from `src/main.cpp`)
 
 **Benchmark Executables:**
 - `bench_arena_allocator` - Arena allocator performance benchmarks
 - `bench_commit_request` - JSON parsing performance benchmarks
 - `bench_parser_comparison` - Comparison benchmarks vs nlohmann::json and RapidJSON
+- `bench_metric` - Metrics system performance benchmarks
 
 **Debug Tools:**
 - `debug_arena` - Debug tool for arena allocator analysis

todo.md

@@ -23,7 +23,7 @@
 - [ ] Implement `DELETE /v1/retention/<policy_id>` endpoint for retention policy removal
 
 ### Infrastructure & Tooling
-- [ ] Implement thread-safe Prometheus metrics library and serve `GET /metrics` endpoint
+- [x] Implement thread-safe Prometheus metrics library and serve `GET /metrics` endpoint
 - [ ] Implement gperf-based HTTP routing for efficient request dispatching
 - [ ] Implement HTTP client for S3 interactions
   - [ ] Design `HttpClient` class following WeaselDB patterns (factory creation, arena allocation, RAII)