[STORM-254] one Spout/Bolt can register metric twice with same name in different timeBucket

In a Bolt's prepare method, we can register metrics twice with the same name, using different timeBucketSizeInSecs parameter, like this:

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public void prepare(Map stormConf, TopologyContext context)
{ 
    mapper = new ObjectMapper(); 
    CountMetric cMetric = new CountMetric(); 
    context.registerMetric("JavaBoltCount", cMetric, 120); 
    CountMetric ccMetric = new CountMetric(); 
    context.registerMetric("JavaBoltCount", ccMetric, 60); 
}

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This is caused by TopologyContext's registerMetric. 
In TopologyContext, all registered metrics holds in a map defined below:
private Map < Integer, Map < Integer,  Map < String, IMetric>>> _registeredMetrics;
timeBucketSizeInSecs ----> __taskId ----> metricName ----> metirc
But the same name check just at the innermost layer.