Fix a race condition in the http instrumentation.

computeApproximateRequestSize is run in a goroutine, but the
handlerFunc that runs in parallel may modify the URL, which is also
needed by computeApproximateRequestSize. So get the URL length
beforehand.

Change-Id: Idb84735845afe7be4ef79b3d642d5764f6d26a7c
Change Fingerprints to be simple uint64s.
Deal sensibly with missing metric names.
Enable finer-grained time resolution.