4.0.0-rc2: put the prometheus exporter on a different port

[matthewdarwin]

When you put plugin = eosio::prometheus_plugin it adds a new /v1/prometheus/metrics URL.

You want this URL to be available to the monitoring system.
But this same port is used for the producer plugin which needs to be highly restricted (and definately not available to the monitoring system).

For security reasons, the prometheus export must run on a dedicated port. Otherwise node operators need to have a proxy server in between to handle the security. This is nonsensical requirement to use a monitoring plugin.

Look at any other blockchain software that has prometheus feature. Always runs on a separate port.

[spoonincode]

I'm moving this out of actionable to discussion because maybe we want to shoehorn in something special for prometheus in 4.0, but maybe we need to reflect on this a little more as the "run such-and-such-plugin on a separate port" is such a common request (#88, eosnetworkfoundation/mandel#343, eosnetworkfoundation/mandel#360, etc etc). I'd like some consideration given before rushing in to anything that may be short sighted.

[bhazzard]

This is a missed requirement on my part. @heifner can you propose a solution here and have @huangminghuang immediately start work on it for inclusion in rc3?

We need a solution that is trivial to implement (ideally that can be designed, developed, tested & reviewed in about 1 week).

If there is such a trivial solution that addresses Matt's point about the need for a more general solution, please propose it.

Otherwise, please propose a focussed solution to the immediate problem at hand. Consider how we might move towards a more generalized solution as part of our focussed solution.