OTLP Exporter must default to http/protobuf

[cijothomas]

The current default in OTLP Exporter is grpc : https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-otlp/Cargo.toml#L64

This must be changed to use http/protobuf, as perf https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specify-protocol

[cijothomas]

Given #1738 is too close, I'd prefer to do it after that release.

[cijothomas]

Tagging for 0.28.0 as this must be done before stable release. This is also a pre-req for #2386

OTLP Exporter should default to HTTP/protobuf and also use request::Blocking by default, as we'll be defaulting to BatchProcessor/PeriodicReader which uses own thread and hence must use reqwest::Blocking client.