I am not very sure about it but since decoding of video is not in exoplayer code, it cannot determine if a frame is late, very late or should be dropped when tunneling mode is enabled

The tunneling decoder reports rendered frames with the callback MediaCodec.OnFrameRenderedListener.onFrameRendered which calls onProcessedTunneledBuffer which updates renderedOutputBufferCount but not the drop counter.
onProcessedTunneledBuffer could try to guess if a frame was not rendered by comparing the received timestamps to the expected ones.

As the tunneling decoders are not required to call onFrameRendered for every frame (and I have not tested if they do either). So it's not sure if the resulting metrics will be reliable.

Unfortunately I will not access to a tunneling setup until the London office reopens (COVID-19 confinement).