Identify BQ Write client to server

[dbolduc]

We need to set the trace_id field to identify our library. Probably something like: "rust-streamwriter" plus some other identifying information.

https://github.com/googleapis/googleapis/blob/2622f170ca5441590b472785b230eac029c67ab5/google/cloud/bigquery/storage/v1/storage.proto#L527-L529

Googlers can see the spec at: https://docs.google.com/document/d/1BlnBv_ijvpDcq9kaH-wQF-INu_QFfdfJpPxQfwk6hxg