Fix metrics export

Actually register the right prometheus Handler. Also make the server
wait for a metrics collection cycle to complete before starting the
HTTP server, so we never serve missing metrics.
3 jobs for master in 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Src
passed #1155
build:src

00:00:11

 
  Build Pkg
passed #1156
build:pkg

00:00:24

 
  Upload
passed #1157
upload:pkg

00:00:14