Fix a race condition in the http instrumentation.
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

m3db/prometheus_client_golang,prometheus/client_golang,telendt/client_golang,stapelberg/client_golang,asbjornenge/client_golang,KevinPike/client_golang,fabxc/client_golang,jimmidyson/prometheus-client_golang,mattkanwisher/client_golang