prometheus/promhttp: actually observe values in trace in example
prometheus/promhttp: actually observe values in trace in example

This is similar to
https://github.com/prometheus/client_golang/pull/535 fixing ExampleInstrumentRoundTripperDuration.

Signed-off-by: Sergiusz Urbaniak <7657f3a0c6319088c9e0bc4813152bb1f5637169@gmail.com>

m3db/prometheus_client_golang,prometheus/client_golang