bigtable: release v1.2.0
bigtable: release v1.2.0

Change-Id: I332651a85265e8cef0cfa7124f6744b82e5122a4
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/50351
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>

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