* Record subscribe stream start time in prometheus.
Adjust for mutes, i. e. take the last unmute time as the start point and
calculate time till the first byte is sent.
* close the tiny window of race
* Prevent long tail sample when publisher glitches.
Thanks to @milos-lk for this.
Publisher restarting would have reset the layer and would have caused a
sample with very high stream start time. We only need to capture when we
do a dummy start or when the state is seeded to a different node upon
migration.
* reduce a diff
* test
* changed the wrong thing, thank you Devin
* Tests for down stream packet push.
A recent issue (padding bit in RTP header) surfaced a gap which slipped
through due to lack of tests. Changes in pion/rtp were not adopted
properly.
So, adding some tests (thank you Claude for the heavy lifting) to test
the down stream packet path using the whole pion chain.
Split out some interfaces so it is easier to have it all in one place
and create fakes.
Will help adding more tests, for example include the upstream path also
in the integration test. May have to create more interfaces and make
things testable, but this is a start.
* missed file
* rtx specific test
