Create an exporter for Cloud Trace

[coryan]

We would like to export traces to Cloud Trace. We need a new crate (maybe google-cloud-otel).

[fiadliel]

There is a current OTel implementation (but based directly on Tonic, and some pre-existing auth crates) here: https://github.com/open-telemetry/opentelemetry-rust-contrib/tree/main/opentelemetry-stackdriver - you may want to liaise with the group if planning an official replacement.

It probably wouldn't take much to convert it - the most annoying piece is probably just detecting the environment-local project ID, if you want to keep that feature.

Btw, I created a completely separate tracing implementation for fastrace here: https://github.com/fiadliel/fastrace-google-cloud (though not published to crates.io at this time). It might at least be useful for another point of view in considering what the tracing features should look like.

[coryan]

"Stackdriver", I forgot to search for that. Thanks! This is an area where replacing existing things may not help that much, or may actually hurt.

[dbolduc]

Also, Googlers should look at go/otel-gcp-trace-exporter-spec to make sure we cover everything the exporter is supposed to do.

[coryan]

@westarle FYI