Add instrumentation to the User cache

So we can monitor its effectiveness and validate design choices.
3 jobs for distrib in 4 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Pkgsrc
passed #11320
build_pkgsrc:stretch

00:00:16

 
  Build Pkg
passed #11321
amd64
build_pkg:stretch:amd64

00:01:42

passed #11322
arm64
build_pkg:stretch:arm64

00:04:04