Additional internal context: b/185842996

Request routing relies on a metadata header being present, but because of the bidi nature
library generators don't automatically attach the write stream metadata
into x-goog-request-headers.

For this API, the stream ID is constant for the whole stream so we
inject it when opening the stream, which is when routing needs the
information.

This causes some minor changes to how we do stream (re)open because
we need to pass in the stream ID as part of the function.  This change
also updates integration testing so that we're testing in an explicit,
non-default region (us-east1).