Deflake metrics test

[jschaul]

example case where this test failed: https://concourse.ops.zinfra.io/teams/main/pipelines/staging/jobs/test/builds/342

output of failing test:

  metrics
    prometheus:                                                                              OK (0.02s)
    work:                                                                                    FAIL (1.06s)
      Error message: /login was called twice
      expected: 2
       but got: 3

      CallStack (from HasCallStack):
        assertFailure, called at ./Test/Tasty/HUnit/Orig.hs:86:32 in tasty-hunit-0.10.0.3-KJER1RJhmod6e0raY4U8z6:Test.Tasty.HUnit.Orig
        assertEqual, called at test/integration/API/Metrics.hs:78:12 in main:API.Metrics
      Use -p '(!/turn/&&!/user.auth.cookies.limit/)&&/metrics.work/' to rerun this test only.

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines