net-mgmt/prometheus2: Direct commit to correct GH_TUPLE

When cherry-pick'ing 2.26.1, Makefile.modules was also incorrectly
merged. This is a direct commit to 2021Q2 to fix the build.

Fixes: 656940c421cf8df146a941c6f2552f6145f143be
Fixes: ece647d08c3933df136bef37559c6987eec192a7
Reported by: phyber (irc)
Approved by: lwhsu (mentor, implicit), portmgr (implicit, fix build)