Approximate total storage space used by all rows in the table which precede
row_key. Buffering the contents of all rows between two subsequent
samples would require space roughly equal to the difference in their
offset_bytes fields.
Sorted streamed sequence of sample row keys in the table. The table might
have contents before the first row key in the list and after the last one,
but a key containing the empty string indicates "end of table" and will be
the last response given, if present.
Note that row keys in this list may not have ever been written to or read
from, and users should therefore not make any assumptions about the row key
structure that are specific to their use case.
