Adds the given count to the current count for the given key. If the key hasn't been seen before, it is assumed to have count 0, and thus this method will set its count to the given amount. Negative increments are equivalent to calling
decrementCount.
To more conveniently increment the count by 1, use {@link #incrementCount(Object)}.
To set a count to a specific value instead of incrementing it, use {@link #setCount(Object,int)}.