Add getGauge(String group, MetricName name) to MetricManager

[ralf0131]

After you register a Gauge to MetricManager using com.alibaba.metrics.MetricManager#register, there is no way to query it from MetricManager.

As an alternative, you can use MetricManager.getIMetricManager().getGauges() to get all the gauges, but it is better to provide a more simple way.