Make opentelemetry optional

Right now opentelemetry and co are [unconditional dependencies](https://github.com/google/tarpc/blob/main/tarpc/Cargo.toml#L61).

I use tarpc in a project where I just communicate between two services over stdio; I mostly need it for really convenient abstractions and proc macro. Thus, opentelemetry is an overkill for me, and it's basically a dead dependency that I need to worry about.

It would be great if it was put under a feature and made optional.

BTW -- thank you for a great library! I've checked a lot of lightweight RPC implementations in rust and this was the most convenient by far. ❤️

Start in tarpc/Cargo.toml at the unconditional opentelemetry dependency and inspect how Cargo features expose optional dependencies. Verify that the crate builds without the dependency enabled, while existing OpenTelemetry support remains available when its feature is enabled.

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