Emit header for gRPC messages

[pcostell]

gRPC connections currently don't have enough information in the request for us to do efficient traffic management/routing.

Will you add a header that allows for us to optimize for this on the server? The header format is:

name: google-cloud-resource-prefix
value: projects/my-project-id

In general, the prefix would be the same as the JSON-over-http URL up to any custom method. For example, a Cloud Datastore commit message over JSON would hit the url projects/my-project-id:commit.

Here is an example usage in the cloud-bigtable-client: https://github.com/GoogleCloudPlatform/cloud-bigtable-client/pull/915/files#diff-1078787e69bc1d7c5bb8507f0a87a143R83

[dhermes]

@pcostell Sorry I never saw this. For some reason GitHub stopped subscribing me to notifications.

[dhermes]

Closing as duplicate of #2230

[tseaver]

@dhermes This issue seems to ask for these headers for all gRPC requests, not just those for the Datastore API.

[dhermes]

When @pcostell says "us" he means the datastore API team.

[tseaver]

But his example points at the Java Bigtable client.

[dhermes]

@pcostell can you clarify when this should apply?

[dhermes]

@pcostell Bump. Can you clarify if you just meant for Datastore or for all APIs that use gRPC?

[pcostell]

This is/will be important for all APIs.