Adding NullStatsReporter and TestScope

[ravirajj]

• Created NullStatsReporter to report no metrics when it is used by ScopeBuilder.
• Created TestScope to snapshot metrics for tests.

[inetchitailo]

I think you just need to add a single if statement:

if (!(reporter instanceof NullStatsReporter) {
 scheduler.scheduleWithFixedDelay(scope.new ReportLoop(uncaughtExceptionHandler), 0, interval.toMillis(), TimeUnit.MILLISECONDS); 
}

[guscastro]

This interface needs to be public, so that the Snapshot can be consumed externally.

[guscastro]

How would TestScope be consumed? ScopeBuilder returns a Scope, which happens to be ScopeImpl at the moment. Is the plan for consumers to cast that to TestScope where needed? Should we have a separate ScopeBuilder that returns a TestScope explicitly? (maybe inheriting from RootScopeBuilder)