Possible race condition could happen if another thread put value into 'metricsCache' by the same key.

        /* It is possible that since this reference object has been
        /* It is possible that since this reference object has been
         * enqueued, that a new metrics has been put into the table
         * enqueued, that a new metrics has been put into the table
         * for the same key value. So we'll test to see if the table maps
         * for the same key value. So we'll test to see if the table maps
         * to THIS reference. If its a new one, we'll leave it alone.
         * to THIS reference. If it's a new one, we'll leave it alone.
         * It is possible that a new entry comes in after our test, but
         * it is unlikely and if this were a problem we would need to
         * synchronize all 'put' and 'remove' accesses to the cache which
         * I would prefer not to do.
         */
         */
        public void dispose() {
        public void dispose() {
            if (metricsCache.get(key) == this) {
            metricsCache.remove(key, this);
                metricsCache.remove(key);
            }
        }

We can use `ConcurrentHashMap.remove(key, value)` method to avoid it.