Put back clock for 'Send metrics to Graphite' test to check logging detail

[fenglish]

This commit 54bf44c removed sinon clock from Send metrics to Graphite test because an error is thrown when mitm version was updated .

  1 failing

  1) Logging to graphite
       Send metrics to Graphite:
     Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/home/circleci/project/build/test/unit/client.spec.old.js)

https://circleci.com/gh/Financial-Times/next-metrics/2246

Somehow clock in the test causes the error. The test needs to check Logging to graphite. It is not super important to know what exactly is the log at this point. The clock is deleted to solve the error to able to release new version of next-metrics.

However, the time stamp by sinon clock in the log in the test is nice to have.
It may be resolved by updating sinon version.

All detail is in this PR => #337.