646: Provided wrapper for histogram

[gkatzioura]

Purpose

Linked issue: close #646

Brief change log

Increased coverage for FlinkCounter, FlinkGauge, FlinkHistogram, FlinkHistogramStatistics, FlinkMeter.

Tests

FlinkCounterTest, FlinkHistogramStatisticsTest, FlinkHistogramTest, FlinkMeterTest, FlinkMetricsITCase

API and Format

Documentation

I initially went for the direction of increasing the coverage through FlinkMetricsITCase,
Yet could not find ways to invoke the creation of Counter or the methods missing coverage from the other metric classes.

I went to implement tests per metric.

Since those classes are wrapper classes (FlinkCounter, FlinkGauge FlinkHistogram FlinkMeter) I proceeded on testing their proxy behaviour.
I created WrapperMetricsTestSuite which lists all the public methods expected to be proxied and invokes those methods.
Onwards it checks if the underlying wrapped metric instance has been invoked for the corresponding method.

Another option was to test each class and verify invocations for every public method, for example:

        Counter counter = mock(Counter.class);
        FlinkCounter flinkCounter = new FlinkCounter(counter);
        flinkCounter.inc();
        verify(counter, times(1)).inc();
        flinkCounter.inc(1);
        verify(counter, times(1)).inc(1);

Can switch to this direction if requested.

The FlinkHistogramStatistics class was not used.
I proceeded to create an instances of it during the initialization of FlinkHistogram, thus wrapping the original statistics instance.
Whenever FlinkHistogram.getStatistics is called it returns back the Wrapper instance of FlinkHistogramStatistics.

[wuchong]

Thanks, @gkatzioura, for your contribution! The current test only verifies the success of the proxy invocation but does not validate the logic of the methods themselves. I believe a better approach would be to test each class and method individually like how Flink does:

https://github.com/apache/flink/tree/bfe359c3e52ded31fb0673b48baece7c53b5e65b/flink-metrics/flink-metrics-dropwizard/src/test/java/org/apache/flink/dropwizard/metrics