Add support for configuring statsd prefix

[petere]

I was looking for a way to configure the statsd prefix used by this agent. Currently, it is hardcoded to use the full host name, but that does not work for me, because it needs to fit into a larger scheme of how statsd and graphite metrics are organized.

Here is a hackish patch that allows specifying the prefix as part of the agent arguments, like localhost:8125@60/myprefix. I'm not fond of this syntax, it's just a demo. Maybe this should be put into a more standard and flexible syntax like a URI?

Comments?

[rkuris]

This is important to us also! Originally, we wanted a prefix of 'cassandra.clustername' where clustername is a munged version of the name of the cluster. It's a challenge to get the clustername before the configuration is parsed, and I wasn't super keen on parsing the Cassandra configuration file itself.

I like the URL idea a lot. I think the crazy syntax originally started from host:port but then we needed an interval, and I didn't like the question mark because you'd have to escape it. Maybe //hostname:port/prefix@60 or //hostname:port/prefix#60 would be better?

I'm open to anything that (a) makes it incrementally better (b) maintains backward compatibility (at least for one release); and (c) includes tests so it doesn't break in the next release.