Add gRPC transport option for OTLP exporter

[Nancy-Chauhan]

Feature Request\n\nThe OTLP exporter currently only supports HTTP transport. For latency-sensitive workloads, gRPC would be significantly more efficient.\n\n## Requirements\n\n- Support OTLP/gRPC as an alternative to OTLP/HTTP\n- Reuse the same exporter interface (export() method)\n- Optional dependency (grpcio) — don't require it for HTTP-only users\n- Support for gRPC metadata (auth headers)\n- TLS configuration\n\n## Technical notes\n\n- The opentelemetry-exporter-otlp-proto-grpc package provides the proto definitions\n- We already have the [grpc] extra in pyproject.toml\n- Should share the retry logic from utils/retry.py