Add prometheus prefix solution

[wirednkod]

Fixes #974

[l0r1s]

Some contestable remarks but lgtm 👍

[l0r1s]

How about this ?

Suggested change

	if (nodeGroup.prometheus_prefix) {
	node.prometheus_prefix = nodeGroup.prometheus_prefix;
	} else if (config.relaychain.prometheus_prefix) {
	node.prometheus_prefix = config.relaychain.prometheus_prefix;
	}
	node.prometheus_prefix = nodeGroup.prometheus_prefix || config.relaychain.prometheus_prefix;

[wirednkod]

not really; cause there is a chance that none of them exist, and in that case I want to avoid adding the key "prometheus_prefix" to the object

[l0r1s]

If prometheus_prefix is some value || undefined it will stay undefined

Suggested change

	if (node.prometheus_prefix)
	nodeSetup.prometheusPrefix = node.prometheus_prefix;
	nodeSetup.prometheusPrefix = node.prometheus_prefix;

[wirednkod]

Not really - prior to this if the key prometheusPrefix does not exist; With your suggestion, it will create the key and "assign" undefined to it

[l0r1s]

👍

[pepoviola]

thanks @wirednkod, looks good! I think we can take another approach by set the prefix (with substrate as default) if not set by config, this will save us some if statements down the road :)
If you are agree, I can make the modifications.

Thanks!