Add example with tracing_opentelemetry and tonic

[swilcox3]

This example shows a more advanced setup and integration with tracing. I think this would be helpful to show a more realistic production environment.
One concern I have is that the example Cargo.toml has to rely on the crates.io version of opentelemetry, because tracing-opentelemetry relies on that crates.io version as well. Maybe this example should be in the tracing-opentelemetry repository, but I couldn't find that.

[jtescher]

Thanks! Probably good idea to show a tracing-opentelemetry example in this repo. (The tracing-opentelemetry source is now in the tracing repo https://github.com/tokio-rs/tracing/tree/master/tracing-opentelemetry). But the example there would have a similar issue as tonic relies on the crates.io release of tracing.

This looks good, still waiting on an admin to fix the builds or rust nightly to ship with rustfmt.