MINOR: promex: expose the master and worker IDs as labels

MINOR: promex: expose the master and worker IDs as labels

This adds support for emitting the master id when defined (when master
is running) and the worker id. Note that we add a check for the value
being a non-empty string, which remains valid for the few other field
that is always defined (node), so no impact on existing fields.

The output looks like this:

  $ curl -s 0:8181/metrics?metrics=haproxy_process_master_id,haproxy_process_worker_id
  haproxy_process_master_id{master_id="..."} 1
  haproxy_process_worker_id{worker_id="..."} 1