MP Metrics - sort alphabetically /metrics output

When playing with https://github.com/wildfly/wildfly/pull/11949 I got upset about metrics not being sorted alphabetically or at least grouped together.

My use case was to look at wildfly_undertow_** metrics via browser.

base: and vendor: metrics are grouped together, WF subsystem metrics not - e.g. wildfly_undertow_** metrics were listed on 4 different places.
If alphabetical order is not easy to be achieved, subsystem metrics should be at least grouped together.

I know machines do not care about the order, but people have to look at /metrics too (from time to time ...).