DBA_TABLESPACE_USAGE_METRICS is empty For one of my 11.2g databases, select * from dba_tablespace_usage_metrics returns no rows. 

Why is that and how can I fix it?

Thank you in advance.
Typically caused by:

Bug 18921384 - DBA_TABLESPACE_USAGE_METRICS VIEW DOES NOT RETURN ROWS WHEN DB_FILES LIMIT IS REACHED

To verify:

show parameter db_files

select count(*) from v$datafile;


Then increase db_files if needed or upgrade to a newer version where it is fixed.

Another possible cause:

Bug 16317020 - Records missing from dba_tablespace_usage_metrics (Doc ID 16317020.8)

Also fixed in newer version, and one-off patch is also available.