Allow users to add dimensionsfor storm metrics

We have encountered a use-case that users want a better control over metrics.

For example, a kafka spout may stream in data from multiple kafka partitions. And the partition list could be dynamic. For a simple count metric, our users might want to a better fine-grained group like grouping the counts by kafka partition id.

It would be more convenient if storm could allow users to add dimensions to individual yamas metrics