bigtable: fix integration test by sending smaller message.
bigtable: fix integration test by sending smaller message.

The gRPC max message size was changed to 4 MB.

Change-Id: I98757c22a86ea6eb0d10b64d788b26301dc94afd
Reviewed-on: https://code-review.googlesource.com/5434
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go