Search widgets now provide tag suggestions once the metric has been determined

Re: #25

Some points to note:

* The way we get the tags is through a token 1 hour query of the metric. This makes me cringe a little but works OK in practice for our metrics.
* The caching is done via a global javascript Object. Each time an user refreshes the page / navigates to a new one, this cache is reset. Not sure if using sessionStorage is a better call.