Add comprehensive godoc documentation to all packages

- Add package-level documentation with usage examples and architecture details
- Document all public types, functions, and methods following godoc conventions
- Remove unused logger.Error type and NewError function
- Apply consistent documentation style across all packages

Packages updated:
- apitls: TLS certificate management with automatic renewal
- config: Environment-based configuration system
- config/depenv: Deployment environment handling
- ekko: Enhanced Echo web framework wrapper
- kafka: Kafka client wrapper with TLS support
- logger: Structured logging with OpenTelemetry integration
- tracing: OpenTelemetry distributed tracing setup
- types: Shared data structures for NTP Pool project
- xff/fastlyxff: Fastly CDN IP range management

All tests pass after documentation changes.

tracing/tracing.go

@@ -1,3 +1,36 @@
+// Package tracing provides OpenTelemetry distributed tracing setup with OTLP export support.
+//
+// This package handles the complete OpenTelemetry SDK initialization including:
+//   - Trace provider configuration with batching and resource detection
+//   - Log provider setup for structured log export via OTLP
+//   - Automatic resource discovery (service name, version, host, container, process info)
+//   - Support for both gRPC and HTTP OTLP exporters with TLS configuration
+//   - Propagation context setup for distributed tracing across services
+//   - Graceful shutdown handling for all telemetry components
+//
+// The package supports various deployment scenarios:
+//   - Development: Local OTLP collectors or observability backends
+//   - Production: Secure OTLP export with mutual TLS authentication
+//   - Container environments: Automatic container and Kubernetes resource detection
+//
+// Configuration is primarily handled via standard OpenTelemetry environment variables:
+//   - OTEL_SERVICE_NAME: Service identification
+//   - OTEL_EXPORTER_OTLP_PROTOCOL: Protocol selection (grpc, http/protobuf)
+//   - OTEL_TRACES_EXPORTER: Exporter type (otlp, autoexport)
+//   - OTEL_RESOURCE_ATTRIBUTES: Additional resource attributes
+//
+// Example usage:
+//
+//	cfg := &tracing.TracerConfig{
+//		ServiceName: "my-service",
+//		Environment: "production",
+//		Endpoint:    "https://otlp.example.com:4317",
+//	}
+//	shutdown, err := tracing.InitTracer(ctx, cfg)
+//	if err != nil {
+//		log.Fatal(err)
+//	}
+//	defer shutdown(ctx)
 package tracing
 
 // todo, review:
@@ -43,34 +76,68 @@ var errInvalidOTLPProtocol = errors.New("invalid OTLP protocol - should be one o
 
 // https://github.com/open-telemetry/opentelemetry-go/blob/main/exporters/otlp/otlptrace/otlptracehttp/example_test.go
 
+// TpShutdownFunc represents a function that gracefully shuts down telemetry providers.
+// It should be called during application shutdown to ensure all telemetry data is flushed
+// and exporters are properly closed. The context can be used to set shutdown timeouts.
 type TpShutdownFunc func(ctx context.Context) error
 
+// Tracer returns the configured OpenTelemetry tracer for the NTP Pool project.
+// This tracer should be used for creating spans and distributed tracing throughout
+// the application. It uses the global tracer provider set up by InitTracer/SetupSDK.
 func Tracer() trace.Tracer {
 	traceProvider := otel.GetTracerProvider()
 	return traceProvider.Tracer("ntppool-tracer")
 }
 
+// Start creates a new span with the given name and options using the configured tracer.
+// This is a convenience function that wraps the standard OpenTelemetry span creation.
+// It returns a new context containing the span and the span itself for further configuration.
+//
+// The returned context should be used for downstream operations to maintain trace correlation.
 func Start(ctx context.Context, spanName string, opts ...trace.SpanStartOption) (context.Context, trace.Span) {
 	return Tracer().Start(ctx, spanName, opts...)
 }
 
+// GetClientCertificate defines a function type for providing client certificates for mutual TLS.
+// This is used when exporting telemetry data to secured OTLP endpoints that require
+// client certificate authentication.
 type GetClientCertificate func(*tls.CertificateRequestInfo) (*tls.Certificate, error)
 
+// TracerConfig provides configuration options for OpenTelemetry tracing setup.
+// It supplements standard OpenTelemetry environment variables with additional
+// NTP Pool-specific configuration including TLS settings for secure OTLP export.
 type TracerConfig struct {
-	ServiceName string
-	Environment string
-	Endpoint    string
-	EndpointURL string
+	ServiceName string // Service name for resource identification (overrides OTEL_SERVICE_NAME)
+	Environment string // Deployment environment (development, staging, production)
+	Endpoint    string // OTLP endpoint hostname/port (e.g., "otlp.example.com:4317")
+	EndpointURL string // Complete OTLP endpoint URL (e.g., "https://otlp.example.com:4317/v1/traces")
 
-	CertificateProvider GetClientCertificate
-	RootCAs             *x509.CertPool
+	CertificateProvider GetClientCertificate // Client certificate provider for mutual TLS
+	RootCAs             *x509.CertPool       // CA certificate pool for server verification
 }
 
+// InitTracer initializes the OpenTelemetry SDK with the provided configuration.
+// This is the main entry point for setting up distributed tracing in applications.
+//
+// The function configures trace and log providers, sets up OTLP exporters,
+// and returns a shutdown function that must be called during application termination.
+//
+// Returns a shutdown function and an error. The shutdown function should be called
+// with a context that has an appropriate timeout for graceful shutdown.
 func InitTracer(ctx context.Context, cfg *TracerConfig) (TpShutdownFunc, error) {
 	// todo: setup environment from cfg
 	return SetupSDK(ctx, cfg)
 }
 
+// SetupSDK performs the complete OpenTelemetry SDK initialization including resource
+// discovery, exporter configuration, provider setup, and shutdown function creation.
+//
+// The function automatically discovers system resources (service info, host, container,
+// process details) and configures both trace and log exporters. It supports multiple
+// OTLP protocols (gRPC, HTTP) and handles TLS configuration for secure deployments.
+//
+// The returned shutdown function coordinates graceful shutdown of all telemetry
+// components in the reverse order of their initialization.
 func SetupSDK(ctx context.Context, cfg *TracerConfig) (shutdown TpShutdownFunc, err error) {
 	if cfg == nil {
 		cfg = &TracerConfig{}