Major Description
Currently metrics in kafka are using old-school JMX directly. This makes adding metrics a pain. It would be good to do one of the following:
1. Convert to Coda Hale's metrics package (https://github.com/codahale/metrics)
2. Write a simple metrics package
The new metrics package should make metrics easier to add and work with and package up the common logic of keeping windowed gauges, histograms, counters, etc. JMX should be just one output of this.
The advantage of the Coda Hale package is that it exists so we don't need to write it. The downsides are (1) introduces another client dependency which causes conflicts, and (2) seems a bit heavy on design. The good news is that the metrics-core package doesn't seem to bring in a lot of dependencies which is nice, though the scala wrapper seems to want scala 2.9. I am also a little skeptical of the approach for histograms--it does sampling instead of bucketing though that may be okay.
