A more useful req object counter

pipe() doesn't actually do anything in request (the metrics code was
take from a router), so we just count the number of req calls for now.

Eg, if req are higher than res data then you are losing requets (or they
are slow)
Instrument the correct thing
Add timings for 4xx and 5xx responses
Allow custom counters
Vary the status code and response times from the example app