spanner: set google-cloud-resource-prefix metadata

[danielnorberg]

Set google-cloud-resource-prefix metadata to speed up spanner data RPCs.

It seems that data RPCs still work without this, albeit with surprisingly reduced performance. I have not found anything in the spanner RPC API docs to indicate which RPCs should have this metadata set but the golang spanner client seems to be setting it for all session creation and data RPCs.

Not sure what is the preferred way to implement this. This PR adds a metadata field on crate::apiv1::spanner_client::Client to avoid having to pass it in to the public methods on the impl.

Other approaches could be to instead explicitly pass in the metadata from call sites, and/or infer it from already available information like database fields on the request types when available.

#323 (comment)

See:

• https://github.com/googleapis/google-cloud-go/blob/7cbffad749a0f8917658406dae73c7fb7151b55d/spanner/client.go#L478
• Use google-cloud-resource-prefix Header googleapis/google-cloud-ruby#1237

[danielnorberg]

Tested the latency of the BeginTransaction and ExecuteStreamingSql RPCs and they also seem to be affected by the presence of google-cloud-resource-prefix same as the session creation RPCs, so it seems prudent to include this metadata for every database-scoped RPC, same as the golang client does.

[yoshidan]

Thank you for your contribution. I will fix lint error

[danielnorberg]

Sorry about the lint error, pushed a fix here: #334