Add ability to filter metrics in DropwizardExports

[vgv]

We are migrating from dropwizard metrics to prometheus right now. We exported all old dropwizard metrics to prometheus and added all metrics to prometheus one by one. However, there is a problem – metric is added to prometheus is exported via DropwizardExports too, so, we see two similar metrics in output.

It will be great if we can filter metrics are exported to Dropwizard by standard MetricFilter mechanism.

[vladimir-bukhtoyarov]

If I understand correctly this fix has not been released. So, bellow is a powerful workaround, that can be used now:

public static MetricRegistry decorateMetricRegistryByFilter(MetricRegistry sourceRegistry, MetricFilter filter) {
        MetricRegistry decoratedRegistry = new MetricRegistry();

        MetricRegistryListener listener = new MetricRegistryListener() {
            @Override
            public void onGaugeAdded(String name, Gauge<?> gauge) {
                if (filter.matches(name, gauge)) {
                    decoratedRegistry.register(name, gauge);
                }
            }

            @Override
            public void onGaugeRemoved(String name) {
                decoratedRegistry.remove(name);
            }

            @Override
            public void onCounterAdded(String name, com.codahale.metrics.Counter counter) {
                if (filter.matches(name, counter)) {
                    decoratedRegistry.register(name, counter);
                }
            }

            @Override
            public void onCounterRemoved(String name) {
                decoratedRegistry.remove(name);
            }

            @Override
            public void onHistogramAdded(String name, Histogram histogram) {
                if (filter.matches(name, histogram)) {
                    decoratedRegistry.register(name, histogram);
                }
            }

            @Override
            public void onHistogramRemoved(String name) {
                decoratedRegistry.remove(name);
            }

            @Override
            public void onMeterAdded(String name, Meter meter) {
                if (filter.matches(name, meter)) {
                    decoratedRegistry.register(name, meter);
                }
            }

            @Override
            public void onMeterRemoved(String name) {
                decoratedRegistry.remove(name);
            }

            @Override
            public void onTimerAdded(String name, Timer timer) {
                if (filter.matches(name, timer)) {
                    decoratedRegistry.register(name, timer);
                }
            }

            @Override
            public void onTimerRemoved(String name) {
                decoratedRegistry.remove(name);
            }
        };
        sourceRegistry.addListener(listener);

        return decoratedRegistry;
    }

Then all we need is passing the decorated registry to the DropwizardExports instead of the original:

...
    MetricRegistry decoratedRegistry decorateMetricRegistryByFilter(originalRegistry, metricFilter);
    DropwizardExports exports = new DropwizardExports(decoratedRegistry, metricFilter);
    ...

Moreover, performance can be dramatically increased by the snippet above in case if the filter works slowly, for example because the filter computes regexp. Because each filter requires to be applied only once, we are free ещ use even inefficient filtering and forget about filtering costs.