Send all metrics w/o DogstatsD

[dabcoder]

• Get rid of the DogStatsD dependency by creating a thread safe counter cache
  • Use a ConcurrentMap to make sure that increments are thread safe - prevent concurrency issue - I may have been over protective with my implementation
  • Use a ReentrantLock to make sure we don't flush and increment the counter cache at the same time (across threads).
  • Use the singleton pattern to only have one instance of the cache in the JVM (also thread safe)
  • Counter are submitted as COUNT with an interval set to 10 s (recommended interval)
• It also adds jenkins.job.started and jenkins.scm.checkout metrics.
• It increases test coverage!
• Wrapping all extensions in global try catch statements so build don't fail because of the plugin - see issue discussed in Provide option to post to local dogstatsd rather than going directly to the API #34 (however, in this PR we are removing DogStatsD dependency)
• Changed the DatadogHttpClient into another singleton - thread safe - Hence we don't have lease logic anymore from the descriptor.

[gzussa]

Didn't check tests at this point. Just the core implementation. Good clean up 👍

[gzussa]

Great. Made comment to push this further.

• Let's manually test this to make sure it work. I have a doubt that resetting the cache before submitting metrics may be a problem.
• Also we should go deeper testing mttr, cycleTime, leadTime metrics. Each deserve their own tests.

[gzussa]

Approved 😆