Add some more frame stats to PCLF

Metrics for calculating sent/recv key frame rate:
num_encoded_frames -- number of encoded frames
num_decoded_frames -- number of decoded frames
num_send_key_frames -- number of sent keyframes
num_recv_key_frames -- number of received keyframes

Metrics for frame size (in bytes) distribution:
recv_key_frame_size_bytes -- key frame size
recv_delta_frame_size_bytes -- delta frame size

The stats are only reported if the user so requests. This is
because the stats may not be useful for regression tracking,
but mainly for adhoc analysis.

Tested:
```
~/s/w/src (frame_stats↑1|✚1) $ autoninja -C out/Debug && out/Debug/test_support_unittests --gtest_filter="PeerConnectionE2EQualityTestSmokeTest.Smoke" 2>/dev/null
Note: Google Test filter = PeerConnectionE2EQualityTestSmokeTest.Smoke
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from PeerConnectionE2EQualityTestSmokeTest
[ RUN      ] PeerConnectionE2EQualityTestSmokeTest.Smoke
...
RESULT num_encoded_frames: Smoke/alice-video= 31 count_biggerIsBetter
RESULT num_decoded_frames: Smoke/alice-video= 31 count_biggerIsBetter
RESULT num_send_key_frames: Smoke/alice-video= 2 count_biggerIsBetter
RESULT num_recv_key_frames: Smoke/alice-video= 2 count_biggerIsBetter
RESULT recv_key_frame_size_bytes: Smoke/alice-video= {1847,126} count_biggerIsBetter
RESULT recv_delta_frame_size_bytes: Smoke/alice-video= {1150.1034,393.66329} count_biggerIsBetter
...
RESULT num_encoded_frames: Smoke/charlie-video= 31 count_biggerIsBetter
RESULT num_decoded_frames: Smoke/charlie-video= 31 count_biggerIsBetter
RESULT num_send_key_frames: Smoke/charlie-video= 2 count_biggerIsBetter
RESULT num_recv_key_frames: Smoke/charlie-video= 2 count_biggerIsBetter
RESULT recv_key_frame_size_bytes: Smoke/charlie-video= {1847,126} count_biggerIsBetter
RESULT recv_delta_frame_size_bytes: Smoke/charlie-video= {1150.1034,393.66329} count_biggerIsBetter
...
[----------] 1 test from PeerConnectionE2EQualityTestSmokeTest (2196 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (2196 ms total)
[  PASSED  ] 1 test.
```

Bug: webrtc:14019
Change-Id: I3d5ea0cf659a6299c6d94f5004cbf0c763b267e5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/260924
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36759}