Returns the number of instances currently borrowed from but not yet
returned to the pool corresponding to the given <code>key</code>.
Returns a negative value if this information is not available.
@param key the key to query
@return the number of instances currently borrowed from but not yet
returned to the pool corresponding to the given <code>key</code>.
Returns the number of instances currently borrowed from but not yet returned to the pool corresponding to the given <code>key</code>. Returns a negative value if this information is not available.
@param key the key to query @return the number of instances currently borrowed from but not yet returned to the pool corresponding to the given <code>key</code>.