MINOR: promex: Expose the global node and description in process metrics

MINOR: promex: Expose the global node and description in process metrics

The global node value is now exposed via "haproxy_process_node" metrics. The
metric value is always set to 1 and the node name itself is the "node"
label. The same is performed for the global description. But only if it is
defined. In that case "haproxy_process_description" metric is defined, with
1 as value and the description itself is set in the "desc" label.