Create Metrics API and default implementation

A decoupling of the metrics library used by several reporting components by providing a similar API in the nifi-api module (under a new package org.apache.nifi.metrics.jvm) will help internalize the dependencies on the yammer/dropwizard metrics libraries and reduce the number of dependencies that NiFi extensions need to collect those types of metrics.

While working towards support for building NiFi on Java 10+, tests for components that use the com.yammer.metrics:metrics-core:2.2.0 dependency are failing due to references to internal JDK modules.  These metrics libraries are now maintained dropwizard, and a new version (4.0.0) has been released that works with Java 10.

The public API for the metrics libraries has changed a bit, and VirtualMachineMetrics is no longer available.  NiFi will supply its own public API for metrics that is derived from dropwizard/yammer's VirtualMachineMetrics to minimize impact to NiFi extensions that currently use this dependency.