key property
Required. The key is a name of one of the Dataset's data columns.
The values of the key (the values in the column) must be in RFC 3339
date-time format, where time-offset = "Z"
(e.g. 1985-04-12T23:20:50.52Z). If for a piece of data the key is not
present or has an invalid value, that piece is ignored by the pipeline.
Implementation
final String key;