Add proper statistics to Pion #3226

Summary

Pion is currently missing some statistics related to various functions. This issue is more like an epic to manage then partition the problem into different parts. Ideally, completing this feature request should introduce no breaking changes.

Currently there are two existing issues related to this:

• Add support for emitting inbound-rtp and media-playout stats from PeerConnection.GetStats #3134 (emitting stats from PeerConnection.GetStats) would ideally be resolved by @drshrey's pr: Add collection for inbound-rtp stats #3221
• Issue accessing ICECandidatePairStats #2779 (a step towards completing ICECandidatePairStats) would be resolved by my pr: Add IceCandidatePairStats ice#809

Here are additional subtasks from @joeturki:

• make the RTCP sender and receiver more easy to use and more automatic [...maybe we should have an easy mode like the javascript api]
  • @Sean-Der suggested spinning up an ReadRTCP internally, until we detect a user has done it themselves.
    • @at-wat Would you happen to have a better idea for this? There is some discussion in the #dev channel in the Pion discord server if you're interested in joining.
• make stats part of pion/interceptor more integrated and easy to use
• easier and cleaner way for people to send media stats when Pion is the receiver
• test pion RTCP more with other clients
• work on the getStats() api, maybe revive and finish Implement PeerConnection GetStats API #610

@joeturki also mentioned that @jech had additional comments related to handling stats in Pion.

Additional context

There is some more discussion that has taken place in the Pion discord server, but this is the gist of the conversation so far.