Summary

• Add docs/guides/EXPORTER_DEVELOPMENT.md covering the complete exporter subsystem
• Document dual pipeline architecture (metrics + traces) with data flow diagrams
• Detail all 7 built-in exporters: Prometheus, StatsD, OTLP Metrics, Jaeger, Zipkin, OTLP Traces, OTLP gRPC
• Document all 3 transport layers (HTTP, gRPC, UDP) with stub/real implementations
• Include custom exporter implementation guide with full code examples
• Document OpenTelemetry compatibility layer and integration patterns
• Provide 4 configuration examples: production multi-backend, local dev, testing stubs, Zipkin custom headers

Resolves #456

Test Plan

• All 9 referenced source files exist and paths are correct
  • metric_exporters.h, trace_exporters.h, opentelemetry_adapter.h, otlp_grpc_exporter.h ✓
  • http_transport.h, grpc_transport.h, udp_transport.h ✓
  • result_types.h, error_codes.h ✓
• All code examples compile conceptually against the actual interfaces
  • Verified interface methods match actual source (export_metrics, export_spans, etc.)
  • Factory functions and config structs match implementations
• Document covers all 6 sections from issue acceptance criteria
  1. Exporter pipeline architecture (Section 2) ✓
  2. All built-in exporters documented (Sections 3-4, 7 exporters) ✓
  3. All 3 transport layers documented (Section 5) ✓
  4. Custom exporter implementation guide (Section 6) ✓
  5. OpenTelemetry integration patterns (Section 7) ✓
  6. At least 3 configuration examples (Section 8, 4 examples) ✓
• Cross-references to COLLECTOR_DEVELOPMENT.md and ARCHITECTURE.md are valid
• No broken markdown links or formatting issues