Add system memory metrics emitted by the JVM

[trask]

Some ideas:

• process.runtime.jvm.system.memory.free
• process.runtime.jvm.system.paging.free

There's also potentially:

• OperatingSystemMXBean#getTotalPhysicalMemorySize
• OperatingSystemMXBean#getTotalSwapSpaceSize
• OperatingSystemMXBean#getCommittedVirtualMemorySize

[jackshirazi]

I agree with Peter that this doesn't seem right (per comment system under process). I don't think these should be part of the stable set of metrics