Using awk in paragraph mode
awk -vRS='\n[ \t]*--' '{print
$1 match($0, $/\(NF *[0-19]+\)/) {print $1, $NFsubstr($0,RSTART,RLENGTH)}
' file
foo-6-25.example.com: ( 49)
foo-5-4.example.com: ( 19)
foo-8-28.example.com: ( 43)
foo-9-7.example.com: ( 91)
foo-5-19.idmz.example.com: ( 19)
foo-7-3.example.com: ( 20)