fix: open connections metrics increment

[alencardc]

Fixes #73

When a new connection was opened, the mobc_pool_connections_opened_total metric was incremented as a gauge, and mobc_pool_connections_open was incremented as a counter. However, mobc_pool_connections_opened_total was described as a counter while mobc_pool_connections_open was described as a gauge. Simultaneously, mobc_pool_connections_open is decremented as a gauge when a connection is closed. Therefore,

• mobc_pool_connections_opened_total was incorrectly registered as a gauge instead of a counter.
• mobc_pool_connections_open was divided into two different metrics, where the gauge represented the negative number of closed connections, and the counter represented the total number of opened connections.

So, this PR increments mobc_pool_connections_opened_total as a counter and mobc_pool_connections_open as a gauge when a new pool connection is opened.

[garrensmith]

@alencardc Nice catch.