Add Auth metrics header

Currently client libraries send metrics header `x-goog-api-client` in API calls. The credential type used for this call needs to be added to the header as well.

@coryan's comment on the topic

> The HTTP + JSON clients will need a few changes. Currently the header is created in the generated code:

https://github.com/googleapis/google-cloud-rust/blob/4fa3e626e6b5bcab34b61a2ad2a2c50ae6d5d72f/src/generated/cloud/kms/v1/src/transport.rs#L56-L59

> We will need to pass the value from the generated code (where the gapic version is available) to the gaxi::http::ReqwestClient, combine this value with the value returned from the credentials, and then keep going.