Add UMA metric for number of RTP transports

This will gather data on whether multiple RTP transports
are in fact used in practice.

Bug: None
Change-Id: I9dd7638106575f10d996fe04e32076ba074b18b5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/416601
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45938}