Update HTTP probe to support latency distributions.
Update HTTP probe to support latency distributions.

Following screenshot shows the results in stackdriver:
http://screen/rP1kffdhvbF

ORIGINAL_AUTHOR=Manu Garg <manugarg@gmail.com>
PiperOrigin-RevId: 175875550

google/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober,google/cloudprober