bigtable: add annotation to ReadRows
bigtable: add annotation to ReadRows

Added result row count to ReadRows
Added read time to annotation

Change-Id: I5c79c15974c5725398ebeff016b404c9e757fe67
Reviewed-on: https://code-review.googlesource.com/24530
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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