OTLP gRPC/Protobuf export support

[miton18]

Currently, pg_tracing exports spans using OTLP HTTP/JSON protocol via the pg_tracing.otel_endpoint parameter (/v1/traces).

It would be great to add support for OTLP gRPC/Protobuf export as an alternative.
This is the format we use at Clever Cloud for our OpenTelemetry infrastructure, and we'd love to use pg_tracing with our existing collectors.

this can bring:

• Lower overhead: Protobuf binary encoding is significantly more compact than JSON, reducing network bandwidth usage — especially relevant for high-throughput PostgreSQL instances generating many spans
• Faster serialization/deserialization: Protobuf parsing is ~5-10x faster than JSON parsing, reducing CPU overhead on both the sender and collector side
• Streaming support: gRPC enables persistent connections and bidirectional streaming, avoiding the overhead of establishing new HTTP connections for each batch
• Industry standard: gRPC is the recommended protocol by OpenTelemetry for production environments when performance matters

Suggested implementation
A new GUC parameter like pg_tracing.otel_protocol with values http/json (default, current behavior) and grpc could control the export format. The gRPC endpoint would typically use port 4317 instead of 4318.

Happy to discuss or contribute if there's interest!