What

This add DC (diagnostics_channel) built-in events, for DC itself. Just like http, console, or process, have built-in events that can be subscribed to, to monitor code, this make it possible for DC to be monitored through DC.

Why

It appeared that multiple people working with DC channels extensively ended up monkey-patching DC in their test app or test suite, to allow some actions to be logged. Now instead of monkey-patching, they can just listen to these events.

It also allows one to create a kind of "dead-letter queue" DC subscription, that will catch all events that were not delivered:

dc.subscribe('diagnostics_channel.publish', ({ channel, data }) => {
  if (channel.hasSubscribers === false) {
    // handle undelivered event
  }
})