CONTRIB: contrib/prometheus-exporter: Add a Prometheus exporter for HAProxy

CONTRIB: contrib/prometheus-exporter: Add a Prometheus exporter for HAProxy

It has been developped as a service applet. Internally, it is called
"promex". To build HAProxy with the promex service, you should use the Makefile
variable "EXTRA_OBJS". To be used, it must be enabled in the configuration with
an "http-request" rule and the corresponding HTTP proxy must enable the HTX
support. For instance:

    frontend test
        mode http
        ...
	option http-use-htx
	http-request use-service prometheus-exporter if { path /metrics }
        ...

See contrib/prometheus-exporter/README for details.