[Bug]: OTEL_EXPORTER_OTLP_PROTOCOL exists but is never used

[lahabana]

What happened?

I'd expect that with this version set and depending on all features to be able to initialise either tonic or http just by leveraging this env var and explecitely set all protocol options.
The defaulting and behaviour dependending on support is pretty clear in the spec

Should there just be a SpanExporter::default() option?

I instead have to do:

        let exporter = match env::var(OTEL_EXPORTER_OTLP_PROTOCOL).unwrap_or(OTEL_EXPORTER_OTLP_PROTOCOL_DEFAULT.to_owned()).as_str() { 
            "http/json" => {
                opentelemetry_otlp::SpanExporter::builder().with_http().with_protocol(Protocol::HttpJson).build()
            }
            "grpc" => {
                opentelemetry_otlp::SpanExporter::builder().with_tonic().build()
            }
            _ => {
                opentelemetry_otlp::SpanExporter::builder().with_http().with_protocol(Protocol::HttpBinary).build()
            }
        }?;

OpenTelemetry API Version (i.e version of opentelemetry crate)

opentelemetry-otlp = { version = "0.31", features = ["metrics", "grpc-tonic", "http-json", "http-proto"] }

OpenTelemetry SDK Version (i.e version of opentelemetry_sdk crate)

opentelemetry_sdk = { version = "0.31.0", features = ["metrics", "rt-tokio", "experimental_metrics_periodicreader_with_async_runtime", "trace"] }

What Exporter(s) are you seeing the problem on?

OTLP

Relevant log output

Tip

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[scottgerring]

Following on from #3230 , we will now honour this environment variable if the feature corresponding to the requested protocol is compiled in, and otherwise log indicating we could not honour the variable, falling back to whatever exporter it does have available, following a basic order of preference.