Add copilot/claude instructions

.github/copilot-instructions.md

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+../CLAUDE.md

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+### Testing
+- Run all tests: `go test ./...`
+- Run tests with verbose output: `go test -v ./...`
+- Run tests for specific package: `go test ./config`
+- Run specific test: `go test -run TestConfigBool ./config`
+
+### Building
+- Build all packages: `go build ./...`
+- Check module dependencies: `go mod tidy`
+- Verify dependencies: `go mod verify`
+
+### Code Quality
+- Format code: `go fmt ./...`
+- Vet code: `go vet ./...`
+- Run static analysis: `staticcheck ./...` (if available)
+
+## Architecture
+
+This is a common library (`go.ntppool.org/common`) providing shared infrastructure for the NTP Pool project. The codebase emphasizes observability, security, and modern Go practices.
+
+### Core Components
+
+**Web Service Foundation:**
+- `ekko/` - Enhanced Echo web framework with pre-configured middleware (OpenTelemetry, Prometheus, logging, security headers)
+- `health/` - Standalone health check HTTP server with `/__health` endpoint
+- `metricsserver/` - Prometheus metrics exposure via `/metrics` endpoint
+
+**Observability Stack:**
+- `logger/` - Structured logging with OpenTelemetry trace integration and multiple output formats
+- `tracing/` - OpenTelemetry distributed tracing with OTLP export support
+- `metricsserver/` - Prometheus metrics with custom registry
+
+**Configuration & Environment:**
+- `config/` - Environment-based configuration with code-generated accessors (`config_accessor.go`)
+- `version/` - Build metadata and version information with Cobra CLI integration
+
+**Security & Communication:**
+- `apitls/` - TLS certificate management with automatic renewal via certman
+- `kafka/` - Kafka client wrapper with TLS support for log streaming
+- `xff/fastlyxff/` - Fastly CDN IP range management for trusted proxy handling
+
+**Utilities:**
+- `ulid/` - Thread-safe ULID generation with monotonic ordering
+- `timeutil/` - JSON-serializable duration types
+- `types/` - Shared data structures (LogScoreAttributes for NTP server scoring)
+
+### Key Patterns
+
+**Functional Options:** Used extensively in `ekko/` for flexible service configuration
+**Interface-Based Design:** `CertificateProvider` in `apitls/` for pluggable certificate management
+**Context Propagation:** Throughout the codebase for cancellation and tracing
+**Graceful Shutdown:** Implemented in web servers and background services
+
+### Dependencies
+
+The codebase heavily uses:
+- Echo web framework with custom middleware stack
+- OpenTelemetry for observability (traces, metrics, logs)
+- Prometheus for metrics collection
+- Kafka for message streaming
+- Cobra for CLI applications
+
+### Code Generation
+
+`config/config_accessor.go` is generated - modify `config.go` and regenerate accessors when adding new configuration options.
+
+## Package Overview
+
+### `apitls/`
+TLS certificate management with automatic renewal support via certman. Provides a CA pool for trusted certificates and interfaces for pluggable certificate providers. Used for secure inter-service communication.
+
+### `config/`
+Environment-based configuration system with code-generated accessor methods. Handles deployment mode, hostname configuration, and TLS settings. Provides URL building utilities for web and management interfaces.
+
+### `ekko/`
+Enhanced Echo web framework wrapper with pre-configured middleware stack including OpenTelemetry tracing, Prometheus metrics, structured logging, gzip compression, and security headers. Supports HTTP/2 with graceful shutdown.
+
+### `health/`
+Standalone HTTP health check server that runs independently from the main application. Exposes `/__health` endpoint with configurable health handlers, timeouts, and graceful shutdown capabilities.
+
+### `kafka/`
+Kafka client wrapper with TLS support for secure log streaming. Provides connection management, broker discovery, and reader/writer factories with compression and batching optimizations.
+
+### `logger/`
+Structured logging system with OpenTelemetry trace integration. Supports multiple output formats (text, OTLP) with configurable log levels, systemd compatibility, and context-aware logging.
+
+### `metricsserver/`
+Dedicated Prometheus metrics HTTP server with custom registry isolation. Exposes `/metrics` endpoint with OpenMetrics support and graceful shutdown handling.
+
+### `timeutil/`
+JSON-serializable duration types that support both string parsing ("30s", "5m") and numeric nanosecond values. Compatible with configuration files and REST APIs.
+
+### `tracing/`
+OpenTelemetry distributed tracing setup with support for OTLP export via gRPC or HTTP. Handles resource detection, propagation, and automatic instrumentation with configurable TLS.
+
+### `types/`
+Shared data structures for the NTP Pool project. Currently contains `LogScoreAttributes` for NTP server scoring with JSON and SQL database compatibility.
+
+### `ulid/`
+Thread-safe ULID (Universally Unique Lexicographically Sortable Identifier) generation using pooled monotonic readers with cryptographically secure seeding for high-concurrency environments.
+
+### `version/`
+Build metadata and version information system with Git integration. Provides CLI commands for Cobra and Kong frameworks, Prometheus build info metrics, and semantic version validation.
+
+### `xff/fastlyxff/`
+Fastly CDN IP range management for trusted proxy handling. Parses Fastly's IP ranges JSON file and generates Echo framework trust options for proper client IP extraction.
+
+## Go Development Best Practices
+
+### Code Style
+- Follow standard Go formatting (`go fmt ./...`)
+- Use `go vet ./...` for static analysis
+- Run `staticcheck ./...` when available
+- Prefer short, descriptive variable names
+- Use interfaces for testability and flexibility
+
+### Error Handling
+- Always handle errors explicitly
+- Use `errors.Join()` for combining multiple errors
+- Wrap errors with context using `fmt.Errorf("context: %w", err)`
+- Return early on errors to reduce nesting
+
+### Testing
+- Write table-driven tests when testing multiple scenarios
+- Use `t.Helper()` in test helper functions
+- Test error conditions, not just happy paths
+- Use `testing.Short()` for integration tests that can be skipped
+
+### Concurrency
+- Use contexts for cancellation and timeouts
+- Prefer channels for communication over shared memory
+- Use `sync.Once` for one-time initialization
+- Always call `defer cancel()` after `context.WithCancel()`
+
+### Performance
+- Use `sync.Pool` for frequently allocated objects
+- Prefer slices over arrays for better performance
+- Use `strings.Builder` for string concatenation in loops
+- Profile before optimizing with `go tool pprof`
+
+### Observability
+- Use structured logging with key-value pairs
+- Add OpenTelemetry spans for external calls
+- Include trace IDs in error messages
+- Use metrics for monitoring application health
+
+### Dependencies
+- Keep dependencies minimal and well-maintained
+- Use `go mod tidy` to clean up unused dependencies
+- Pin major versions to avoid breaking changes
+- Prefer standard library when possible
+
+### Security
+- Never log sensitive information (passwords, tokens)
+- Use `crypto/rand` for cryptographic randomness
+- Validate all inputs at API boundaries
+- Use TLS for all network communication