Adds additional metrics for remaining capacity, min, and max

[iainsproat]

This PR adds three additional metrics.

numMin and numMax return the configured min and max values. This allows external services to query the configuration of the pool.

numRemainingCapacity returns the difference between the maximum value and the number of used and pending creation resources. This is particularly useful in determining whether the connection pool is saturated, i.e. there is no additional capacity.