Replace stdout metrics by expvars.

I'm taking a stab at using `expvar` to publish metrics over HTTP, for a metrics agent outside the process to collect. I'd like to have early comments before I start changing it everywhere.

Something I noticed is the metrics need to be global. Otherwise, if the `listTask` holds onto its metrics, `expvar` panics during the tests, saying that the var names are reused. Which makes sense, the variables should have a global, unique and predictable name so that an exterior agent can read them. 

But then it goes in conflict with the previous PR, where I made `listTask` to avoid having global state.

r: @fbogsany, @benbjohnson
If these are `Add` instead of `Set`, then you avoid the assumption that `walkPath` is non-reentrant.