Proper ConcurrentMap semantics

[cfieber]

fix usage of concurrent map for gauge lock creation

[wilkinsona]

Thanks for the PR.

Is there any reason why you've taken your approach as opposed to something like this:

Object lock = this.gaugeLocks.get(name);
if (lock == null) {
    Object newLock = new Object();
    lock = this.gaugeLocks.putIfAbsent(name, newLock);
    if (lock == null) {
        lock = newLock;
    }
 }

The above, I think, will perform better where there's an existing lock as there's a single call to get() and that's it. Notably, it avoids the creation of newLock unless there's a good chance it'll be used. In the case where there isn't an existing lock the above is probably slightly more expensive as there are two interactions with the map. Optimising for the case where the lock already exists feels like the right approach to me. Do you have experience to the contrary?