Add possibility to reset metric value

[rc-mairovichaa]

To reduce time required for test runs application context is reused sometimes in different tests. It is highly relevant for integration tests. By "application context" here could be considered Spring-context.

It's suggested to introduce "reset" method to metrics to be able to start from scratch in a particular test case if it's needed. Also it makes sense to add such method on MetricRegistry level (moreover, maybe it's enough - no need to add for each Metric type).

For example

MetricRegistry registry = new DefaultMetricRegistry();
Counter counter = registry.counter(withName("counter"));

counter.inc();
counter.inc();
counter.forEach(instance -> {
    Long v = (Long)instance.valueOf(COUNT);
    
    // Current value: 2
    System.out.println("Current value: " + v);
});
counter.reset();
// or
registry.resetMetrics();

// Current value: 0
System.out.println("Current value: " + v);