Add OpenTelemetry OTLP exporter with full SDK support

[etiennep]

Summary

Adds production-ready OpenTelemetry Protocol (OTLP) exporter using the official OpenTelemetry SDK with comprehensive support for both gRPC and HTTP/Protobuf transports.

Features

✅ Dual Transport Support: gRPC and HTTP/Protobuf protocols
✅ Environment Variables: Full OTEL_* environment variable support
✅ Resource Detection: AWS (EC2, ECS, EKS, Lambda), GCP, Azure, K8s, host, process
✅ All Metric Types: Counter, Gauge, Histogram with proper semantics
✅ Tag Conversion: Automatic stats tags → OpenTelemetry attributes
✅ Production Ready: Thread-safe, tested, documented

Usage

// Simple - uses environment variables
handler, err := otlp.NewSDKHandlerFromEnv(ctx)
stats.Register(handler)

// Or explicit configuration
handler, err := otlp.NewSDKHandler(ctx, otlp.SDKConfig{
    Protocol: otlp.ProtocolGRPC,
    Endpoint: "localhost:4317",
})

Implementation Highlights

• Gauge Semantics: Uses UpDownCounter with delta calculation to maintain absolute value semantics (workaround until stable OTel SDK adds Gauge)
• Context Handling: Background context for metric recording prevents context cancellation issues
• Performance: Lock-free reads for instrument lookup, efficient caching
• Resource Detection: Automatic cloud provider metadata detection

Documentation

• 📖 Complete OTLP README with examples
• 📝 Implementation Notes explaining design decisions
• 🔧 Example code for all major use cases
• ☁️ Cloud resource detector guides (AWS, GCP, Azure)

Testing

• ✅ Unit tests for all metric types
• ✅ Gauge behavior verification
• ✅ HTTP and gRPC protocol tests
• ✅ Value type conversion tests
• ✅ Performance benchmarks

Changes

• Added otlp/sdk_handler.go - Main OpenTelemetry SDK integration
• Added otlp/sdk_handler_test.go - Comprehensive tests
• Added otlp/example_test.go - Usage examples
• Added otlp/README.md - Complete documentation
• Added otlp/IMPLEMENTATION_NOTES.md - Design decisions
• Updated README.md - Added OpenTelemetry backend overview
• Updated HISTORY.md - Added v5.9.0 release notes
• Updated version/version.go - Bumped to 5.9.0
• Updated otlp/go.mod - Added OpenTelemetry SDK dependencies

Backward Compatibility

✅ Fully backward compatible - This is a new feature addition that doesn't change existing APIs. The legacy otlp.Handler remains available for existing users.

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