Introduce a base "metrics" subsystem and move microprofile-metrics to the microprofile feature-pack

The microprofile-metrics-smallrye subsystem is provided by the wilfdfly galleon-pack.
However its logical inclusion is in the microprofile feature pack (org.wildfly:wildfly-mp-feature-pack-galleon-common) which provides features for MicroProfile.

We should move the subsystem (and extension) to this feature pack.

However, the base WildFly server is still expected to provide "base" metrics (for the WildFly management model and the JVM) that can be consumed out of the box.
I propose to introduce a simple "metrics" subsystem (without any external dependencies) that exposes the WildFly management model and the JVM (using JMX) to the /metrics endpoint (on the management port) with the Prometheus format.

The microprofile-metrics-smallrye subsystem will be refactored to "plug" in the same HTTP endpoint and overrides the base metrics endpoint and provide the full features of MicroProfile metrics (application metrics, JSON output, etc.)