Add stats on queued event drops when apps aren't keeping up with reading

[tonyherre]

If we have batched read APIs (or even ReadableStreams) for getting high frequency data to JS eg received RTP packets, remote acks of sent packets, etc, then the browser will need to keep a buffer of unread events (directly in batch APIs or eg in an UnderlyingSource for a ReadableStream). This buffer will have a bounded size, so there's a risk the JS app doesn't read frequently enough and info gets lost in a way which is indistinguishable from network packet loss.

As suggested by @aboba in #42 (comment), WebTransport has https://www.w3.org/TR/webtransport/#web-transport-datagram-stats.
Adding similar counters of droppedReceivedPackets, droppedAcks etc would be a real benefit for apps.