Sort mbeans returned by /v2/metrics/mbeans
Add leading-uris helper func

This allows us to easily determine all the path of a given URI, for use
in middleware (among other things).
Add multitime! macro

Let's say you'd like to execute some code, but you want to wrap it in a
call to `time!` so that its performance is tracked. That's great, but
what if you want to have its performance tracked in multiple timers
simultaneously (e.g. a more specific timer, and a roll-up timer)?

The `multitime!` macro works exactly like `time!`, except it support
multiple timers.
Support multiple timers in wrap-with-metrics middleware

Normalization functions can now return more than one metric name. If so,
then each of those metrics are updated when the request is executed.
Capture request metrics on per-url/per-path basis

When we migrated to versioned urls, we didn't update our metrics
middleware. Originally, we had urls like /resources, /commands, etc.
We configured the metrics middlware to only take the first component of
the path and create a metric for that, so we had metrics that tracked
all requests to /resources, /commands, etc. and all was right with the
world.

When we moved to versioned urls, though, the first path component became
/v1, /v2, etc. So the metrics are now rolled up at that level, which
is...not very useful at all, actually. It also breaks the documented
API.

I've modified the call to wrap-with-metrics to instead track metrics for
all the leading components of a url. So if you request /foo/bar/baz,
we'll track metrics for /foo/bar/baz, as well as /foo/bar and /foo.
Multiple urls that share leading path components will be treated as
rollups.

Because each url has a corresponding metric, and metrics are exposed via
the /metrics endpoint, to avoid a situation where requesting a metric
creates a new metric (and so on, it's metrics all the way down) I opted
to collapse all metrics requests to a single metric.
Restore compability for v2 metrics urls
Update metrics docs to reflect the new structure
Support mbeans with "/" in their names
Don't collapse metrics requests, for consistency
Fix typo in metrics route
Merge pull request #571 from grimradical/metrics-fixes

(21618) Track HTTP metrics for each URL, restore compatibility with v2 metrics api