Move JVM system metrics out of process.runtime.jvm.*

[trask]

These two metric names feel awkward since they are put system-level metrics under the process.* namespace:

• process.runtime.jvm.system.cpu.utilization
• process.runtime.jvm.system.cpu.load_1m

The JVM runtime metric stability WG would like to explore if these can be made to fit under system.*, e.g.

• system.cpu.recent_utilization (the JVM metric is only a gauge of recent utilization for some vague definition of "recent" and so doesn't meet the definition of system.cpu.utilization
• system.cpu.load

And we would propose for these to be opt-in from the Java Instrumentation perspective.