Add default prometheus metrics to router
Add default prometheus metrics to router

When making a request to the API with the path /metrics
default prometheus exposition format metrics will be returned

Signed-off-by: Toby Lorne <527052641d65eef236eeef9545b2acac74c35d57@digital.cabinet-office.gov.uk>
Co-authored-by: Sebastian <8ea147569987321d46e8a13eb06f2c40c4702251@digital.cabinet-office.gov.uk>

alphagov/router