Backport CsvReporter fixes and MetricName changes from master

The changes in this pull request backport two things from master:

* Modify `CsvReporter` to name files based on `MetricName.toString()` per codahale/metrics@2453bb45eb4cb710784c450c21d50b5dbec6309f
  
  Fixes the issue where the CsvReport fails will multiple metrics with the same name.

* Modify `MetricName.toString` and associated tests to follow the pattern in master (i.e., use group, type, name, and scope for the metric name) instead of the mbean name.

  Makes `.csv` files created by `CsvReporter` easily accessible from the command line, as the previous implementation results in `"` characters in the produced filenames.