add multiprocess capability to PROMETHEUS_METRICS_EXPORT_PORT and _RANGE; fix PROMETHEUS_METRICS_EXPORT_ADDRESS=ANY

[neeels]

running django under uwsgi, this enables correct stats exported via a separate port:

• because of multiple python processes, django under uwsgi needs a PROMETHEUS_MULTIPROC_DIR
• this was only supported for the django view export, so before this patch, django users were forced to export a view on the public web server to export stats (and then guard that URL via nginx.... ugly)
• after these patches, setting PROMETHEUS_MULTIPROC_DIR also has an effect on PROMETHEUS_METRICS_EXPORT_PORT as well as PROMETHEUS_METRICS_EXPORT_PORT_RANGE.
• and after this patch, setting PROMETHEUS_METRICS_EXPORT_ADDRESS="" and =None safely have the same effect of using ANY interface, instead of requiring ="" for a port range and =None for a single port, so confusing.

[neeels]

this accidentally / coincidentally is similar to PR 283, no idea which patch is better.

But apparently maintenance of this repository has ceased, if this problem has not been solved for > 1 year?