Add config to specify the key should be used without prefixing 

Here's the use case:

It makes sense for us (don't know about others) to allow certain parts of the hierarchy to not be spread out by the type of stat.  This patch is a first cut at allowing a configured list of prefixes that, when matched, puts the stat exactly where the "key" says it should go, without e.g. stats.timers being prefixed.  We like this consolidated view.

I'm setting up this pull request to get feedback to see if this is a desirable feature, and if it is, to see what else needs to be added to make this viable for inclusion.