Consider removing tonic-build from opentelemetry-proto build dependencies

[matklad]

At the moment, opentelemetry-proto has tonic-build in its build dependencies:

opentelemetry-rust/opentelemetry-proto/Cargo.toml

Lines 64 to 66 in a57993c

	[build-dependencies]
	tonic-build = { version = "0.8.0", optional = true }
	prost-build = { version = "0.11.1", optional = true }

This means .proto -> .rs compilation happens in the downstream projects which use otl-rust. That is, downstream project:

• should have access to protoc during build
• spend time compiling tonic-build itself, and running it on fixed otl protos
• have more deps in Cargo.lock

An alternative would be to publish otl-proto crate with all Rust code pre-generated. A couple of ways to go about that:

• commit generated code to the repo, with a #[test] which checks that the generated code is fresh (so, move tonic-build from a build-dep to a dev-deps. dev deps, unilke builddeps, don't "infect" downstream consumers)
• run code generation specifically before publishing, such that the generated code gets into the .crate archive sumbitted to crates.io, but not into the git repo.

[djc]

We already do something similar for opentelemetry-stackdriver, I guess we could easily copy that code over.