Expose relative packet arrival delay metric in stats API.

The metric is non-standard and documented in: https://github.com/henbos/webrtc-provisional-stats/pull/14

Bug: webrtc:10333
Change-Id: Ie5b4bbad5b1e2c9104742931529bab8f48f51f8c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125861
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26999}
diff --git a/modules/audio_coding/acm2/acm_receiver.cc b/modules/audio_coding/acm2/acm_receiver.cc
index d296a57..da7d621 100644
--- a/modules/audio_coding/acm2/acm_receiver.cc
+++ b/modules/audio_coding/acm2/acm_receiver.cc
@@ -257,6 +257,8 @@
       neteq_lifetime_stat.jitter_buffer_emitted_count;
   acm_stat->delayedPacketOutageSamples =
       neteq_lifetime_stat.delayed_packet_outage_samples;
+  acm_stat->relativePacketArrivalDelayMs =
+      neteq_lifetime_stat.relative_packet_arrival_delay_ms;
 
   NetEqOperationsAndState neteq_operations_and_state =
       neteq_->GetOperationsAndState();