ParseInstanceKey will parse an InstanceKey from a string representation such as 127.0.0.1:3306 or some.hostname `defaultPort` is used if `hostPort` does not include a port.

ReadThrottleMetric returns replication lag for a given connection config; either by explicit query or via SHOW SLAVE STATUS.

InstanceKey is an instance indicator, identifued by hostname and port.

Probe is the minimal configuration required to connect to a MySQL server.