datastore: add x-goog-api-client header
datastore: add x-goog-api-client header

Change-Id: I0c3cc8870627628be89da6a18400246b02387b5c
Reviewed-on: https://code-review.googlesource.com/11151
Reviewed-by: Brian Watson <608632db7e08b1d2f429d9825017f928dcac62c7@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go