Adds simple form of metrics reporting.

If the Redis client has the option 'metrics' enabled, an event 'stats'
will be emitted per command execution.

The event has a 'command' which has:

 - the name of the Redis command executed
 - a table with its arguments
 - a 'timestamps' table

The 'timestamps' table has the following fields (each one is a value in
nanoseconds):

 - queued: when the command was placed in a queue to be executed
 - sent: when the command was attempted to be sent
 - received: when the response was received and parsed