Improve Class Loading metrics instrumentation

[lenin-jaganathan]

Describe the bug Currently, the JVM class unloading metric is being registered as a Function counter. The description says "The total number of classes unloaded since the Java virtual machine has started execution" which will not be true when this metric is used in a Step Meter Registry.

Environment

• Micrometer version - Any version
• Micrometer registry - Any Step Registry can be used
• OS: Any
• Java version: Any

To Reproduce How to reproduce the bug: Start the java application with the micrometer JVM binder enabled. Use any step registry to register these metrics. After one step is completed, the unloaded class becomes zero.

Expected behavior jvm.classes.unloaded - should represent the total number of classes unloaded since the Java virtual machine has started execution