This is documented as:
> you can query how many buffers are queued by reading the
> #gstqueue:current-level-buffers property. you can track changes
> by connecting to the notify::current-level-buffers signal (which
> like all signals will be emitted from the streaming thread). the same
> applies to the #gstqueue:current-level-time and
> #gstqueue:current-level-bytes properties.
... but was not implemented.
This also respects the `notify::silent` property for the notify signals
to be less intrusive.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7486>
ba94af0285