metrics: add getMetrics helper to acceptance tests
metrics: add getMetrics helper to acceptance tests

so that we can move from a It/To to a Eventually/Should matcher test
style, we need a helper function to getMetrics conveniently

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

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf