OTLP Stabilization: Protocol enum ambiguity in API

[scottgerring]

OpenTelemetry Protocol Requirements:
The OTLP specification supports multiple transport protocols: gRPC, HTTP/protobuf, and HTTP/JSON.

We model this using a fluent builder style API - first to select the transport protocol (note the feature gates):

opentelemetry-rust/opentelemetry-otlp/src/span.rs

Lines 60 to 61 in c5fde17

	#[cfg(any(feature = "http-proto", feature = "http-json"))]
	pub fn with_http(self) -> SpanExporterBuilder<HttpExporterBuilderSet> {

opentelemetry-rust/opentelemetry-otlp/src/span.rs

Lines 68 to 70 in c5fde17

	#[cfg(feature = "grpc-tonic")]
	impl SpanExporterBuilder<TonicExporterBuilderSet> {
	/// Build the [SpanExporter] with the gRPC Tonic transport.

Then, to select the encoding (rather confusingly called "protocol"):

opentelemetry-rust/opentelemetry-otlp/src/exporter/mod.rs

Line 258 in c5fde17

fn with_protocol(mut self, protocol: Protocol) -> Self {

opentelemetry-rust/opentelemetry-otlp/src/lib.rs

Lines 440 to 450 in c5fde17

	/// The communication protocol to use when exporting data.
	#[cfg_attr(feature = "serialize", derive(Deserialize, Serialize))]
	#[derive(Clone, Copy, Debug, Eq, PartialEq)]
	pub enum Protocol {
	/// GRPC protocol
	Grpc,
	/// HTTP protocol with binary protobuf
	HttpBinary,
	/// HTTP protocol with JSON payload
	HttpJson,
	}

This means we can select impossible combinations which are silently remapped at runtime:

HTTP Transport + gRPC Protocol: with_http().with_protocol(Protocol::Grpc) uses HTTP transport with protobuf instead of actual gRPC wire format, silently accepting semantically incorrect configuration.

exporter/http/mod.rs:303 - Protocol::Grpc falls through to _ wildcard and gets HTTP+protobuf instead of failing

gRPC Transport + HTTP Protocol: with_tonic().with_protocol(Protocol::HttpJson) ignores protocol setting entirely, always uses gRPC regardless of specified protocol

exporter/tonic/mod.rs:140 - TonicExporterBuilder hard-codes protocol: Protocol::Grpc regardless of user configuration

Feature-Gated Runtime Behavior: Protocol::HttpJson compiles successfully but silently falls back to protobuf when http-json feature is disabled
exporter/http/mod.rs:298-299 - #[cfg(feature = "http-json")] arm disappears at compile time without the feature