MongoMetricsCommandListener is not thread safe

[jshields-squarespace]

MongoMetricsCommandListener is not thread safe because it uses a shared instance of Timer.Builder. This means that concurrent calls to commandSucceeded() or commandFailed() can generate incorrect metrics.

I've created a unit test here that reproduces the issue: jshields-squarespace@80d0fd6

This test creates 4 threads that execute different Mongo operations, then checks that the metric corresponding to each command has been incremented by 1. The test fails when the threads are run concurrently (concurrent = true), but passes when the threads are run in serial (concurrent = false).

Example concurrent output:

[mongod output] iterationsCompleted=1, insertCommandCount=1, updateCommandCount=2, deleteCommandCount=0, dropIndexesCommandCount=1

org.opentest4j.AssertionFailedError: 
Expecting:
 <2L>
to be equal to:
 <1L>
but was not.

Example serial output:

[mongod output] iterationsCompleted=100, insertCommandCount=100, updateCommandCount=100, deleteCommandCount=100, dropIndexesCommandCount=100

[jshields-squarespace]

I created an example fix in PR #2402, and verified that with it my example test case passes when run concurrently.