Make use of pre-existing context in InstrumentRoundTripperTrace
Make use of pre-existing context in InstrumentRoundTripperTrace

Fixes #580

Signed-off-by: beorn7 <a31805f6001ab3b535ea95133fbc4db46e97e8d1@rabenste.in>

prometheus/client_golang,m3db/prometheus_client_golang