Add telemetry event for LiveView `on_mount` callback

No on_mount telemetry event is emitted for LiveView.on_mount/1 callbacks.

These are the events currently emitted for LiveViews: mount, handle_params, handle_event, render.

Do you think an on_mount event should be added?

Short background: I’m attaching event listeners to enrich AppSignal spans with custom data (user ID, current path). Maybe there’s a better way altogether. If not, being able to instrument on_mount would help provide more context for on_mount errors.