Add backend handler metric req count / resp secs
Add backend handler metric req count / resp secs

We want to measure:
* the number of requests
* the duration of responses

We measure the backend handlers individually, so the metrics are defined
as prometheus vectors

Signed-off-by: Toby Lorne <527052641d65eef236eeef9545b2acac74c35d57@digital.cabinet-office.gov.uk>

alphagov/router