Add tags support

[gbaz]

We use ekg to connect up to datadog, and the latter supports breaking out metrics by "tags" but ekg doesn't seem to offer a way to do this?

It would be nice to add tags support to ekg. I'm open to working on this if the maintainers are up for it, and willing to give a little guidance.

[23Skidoo]

I have nothing against, as long as it doesn't overcomplicate the interface for people who don't need tags.

/cc @tibbe