Allow to add user metrics via run_context

Using run_context, users are able to add metrics sent via the
chef_handler.
It will help to count various event (ignored failures) or measure
specific resource execution (disk format).

Fix #6
This rules! So simple, too. Would you mind using `(` and `)` with the methods so they're uniform with the rest of the code? I could also make the change if you'd like. I'll pull the change down and use it over the holiday and then merge it next week.