Add native prometheus support to manticore daemon

[jsuto]

You recently published a docker based prometheus metric exporter tool using a dockerized php application.

While it works, I believe it would be much better if the manticore search daemon could be configured to enable prometheus metrics on port 9100 or similar.

The reasoning is that such solution wouldn't require an additional component to make the monitoring work, and the less moving parts would align better with the keep it simple philosophy.

Optionally it would be nice to have a whitelist of ip address(es) in the config file for the prometheus nodes to restrict access to the metrics url, eg. prometheus_hosts = 1.2.3.4/32,10.0.0.0/24 or something like that.