the number of pending entries in GIN index with FASTUPDATE=on

Hi,

Is there the way to know the number of pending entries in GIN index which
was created with FASTUPDATE = on? If not, is it worth implementing the
function returning that number?

I sometimes would like to know that number when I measure how much
pending entries affect the performance of GIN index scan and tune how
frequently autovacuum should run VACUUM to clean up them.

Regards,

-- 
Fujii Masao