Return the usage of the pool as value between 0 and 1 (or -1 if the pool is not
limited).
- 1 means that the maximum number of connections have been allocated
- 0 means that no connection is currently active
- -1 means there is not limit to the number of connections that can be allocated
This may also return
null
if the data source does not provide the necessary
information to compute the poll usage.