Returns a function used to generate specific senders (like mailer) that takes three optional named arguments, namely :metric-id :org and :async, that modify which metric the events are sent to.

Specifically, if :metric-id is supplied, every single event is sent to that metric, otherwise the event’s :service field is used to construct the destination Boundary’s metric id. In both cases, organization is prepended if non nil. The last argument, :async, switches the endpoint to the asynchronous one (default is sync).

Examples:

(def bdry (boundary eml tkn))
(when :foo (bdry)) => builds the destination metric id with :service
(when :foo (bdry {:async true})) => same as previous, but async
(when :foo (bdry {:metric-id "METRIC_ID"})) => sends to METRIC_ID