Conditionally send subscription events

@benwilson512 It doesn’t seem to be crashing. If I raise in the subscription resolver, my test process doesn’t crash. I confirmed that they’re the same process.

The middleware approach does raise though with: [error] no function clause matching in Absinthe.Middleware.Telemetry.on_complete/2.

It seems like Absinthe is rescuing exceptions at a high level and not crashing; does that sound right to you?