DiscardSender implements the Sender interface and drops all events.

MockSender implements the Sender interface by retaining a slice of added events, for use in unit tests.

Response is a record of an event sent.

WriterSender implements the Sender interface by marshalling events to JSON and writing to STDOUT, or to the writer W if one is specified.

Metrics is an interface that can be fulfilled by something like statsd.

Sender is responsible for handling events after Send() is called.