id ``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.
google.protobuf.timestamp_pb2.Timestamp
read_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"``.
