CollapserMetrics

[benjchristensen]

The HystrixCollapser received some new functionality in recent months that should be exposed in its own metrics but hasn't yet.

Let's expose a new metrics publisher similar to the ones for Command and ThreadPool.

Example metrics:

• collapser executions
• number of batches
• number of collapsed requests per batch
• number of responses from cache
• number of shards

[mattrjacobs]

Closed by #576