Fix Prometheus exporter

Hopefully by explicitly setting type and help messages the node exporter
will be able to merge values for the same metric when spread across
multiple files.
4 jobs from master in 3 minutes and 21 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build Pkgsrc
passed #6208
build_pkgsrc:stretch

00:00:39

 
  Build Pkg
passed #6209
amd64
build_pkg:stretch:amd64

00:00:28

passed #6210
arm64
build_pkg:stretch:arm64

00:02:06

 
  Upload Pkg
passed #6211
upload_pkg

00:00:35