What units used in DBA_TABLESPACE_USAGE_METRICS?
Blocks. Blocks are the units of the DBA_TABLESPACE_USAGE_METRICS.USED_SPACE and DBA_TABLESPACE_USAGE_METRICS.TABLESPACE_SIZE. The latter column accounts for possible AUTOEXTEND MAXSIZE.

EDIT: I'm not sure what is the meaning of USED_SPACE for undo tablespace though. For example:

SQL&gt; SELECT tablespace_name, sum(blocks), status FROM dba_undo_extents GROUP BY tablespace_name, status ;

TABLESPACE_NAME                SUM(BLOCKS) STATUS
------------------------------ ----------- ---------
UNDOTBS1                               128 ACTIVE
UNDOTBS1                              5312 UNEXPIRED
UNDOTBS1                              8960 EXPIRED

SQL&gt; select USED_SPACE from DBA_TABLESPACE_USAGE_METRICS WHERE TABLESPACE_NAME='UNDOTBS1';

USED_SPACE
----------
      2864

SQL&gt; show parameter block_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_block_size                        integer     8192