Be compatible with IBM JDK in a lenient way

[davidkarlsen]

DefaultExports.initialize() will add register (among other) StandardExports.
StandardExports has two com.sun.management classes.
It will fail on a IBM JDK with:

[1/10/17 10:19:50:691 CET] 00000021 com.ibm.ws.webcontainer.webapp                               E SRVE0283E: Exception caught while initializing context: java.lang.ClassCastException: com.ibm.lang.management.UnixExtendedOperatingSystem incompatible with com.sun.management.OperatingSystemMXBean
        at io.prometheus.client.hotspot.StandardExports.<init>(StandardExports.java:43)
        at io.prometheus.client.hotspot.DefaultExports.initialize(DefaultExports.java:23)

I think it would be better to

1. use java.lang.managment where possible, and where it does not provide the additional metrics we want:
2. use some classload-test to see if the classes are available, and if not, skip registering.

[brian-brazil]

Default exports is in the Hotspot module, and thus doesn't support non-Hotspot JVMs such as IBM's.

use java.lang.managment where possible, and where it does not provide the additional metrics we want:

We do this a but, however this isn't part of the Java spec and thus varies across JVM implementations. Basically we're hardcoding to what Hotspot provides.

use some classload-test to see if the classes are available, and if not, skip registering.

I was informed this wouldn't work, but if you can get it working that'd be great.