doc: add region tags to customer_record.proto so it can be embedded (#391)

In response to customer feedback, where they had trouble finding the
module generated from this file.

Author: tswast

bigquery_storage/snippets/customer_record.proto

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// [START bigquerystorage_append_rows_pending_customer_record]
 // The BigQuery Storage API expects protocol buffer data to be encoded in the
 // proto2 wire format. This allows it to disambiguate missing optional fields
 // from default values without the need for wrapper types.
@@ -26,3 +27,4 @@ message CustomerRecord {
   // Use the required keyword for client-side validation of required fields.
   required int64 row_num = 2;
 }
+// [END bigquerystorage_append_rows_pending_customer_record]