bytes id may be used to identify the transaction in subsequent
Read][google.spanner.v1.Spanner.Read],
ExecuteSql][google.spanner.v1.Spanner.ExecuteSql],
Commit][google.spanner.v1.Spanner.Commit], or
Rollback][google.spanner.v1.Spanner.Rollback] calls.
Single-use read-only transactions do not have IDs, because
single-use transactions do not support multiple requests.
read_timestamp
google.protobuf.timestamp_pb2.Timestamp
For snapshot read-only transactions, the read timestamp
chosen for the transaction. Not returned by default: see
TransactionOptions.ReadOnly.return_read_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.return_read_timestamp].
A timestamp in RFC3339 UTC "Zulu" format, accurate to
nanoseconds. Example: "2014-10-02T15:01:23.045123456Z".
