Add relative_packet_arrival_delay and jitter_buffer_packets_received statistics.

Bug: webrtc:10333
Change-Id: I415e2286b426cbca940fe3a187957531847272ec
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/124780
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26976}

modules/audio_coding/neteq/include/neteq.h

@@ -71,9 +71,18 @@ struct NetEqLifetimeStatistics {
   uint64_t concealment_events = 0;
   uint64_t jitter_buffer_delay_ms = 0;
   uint64_t jitter_buffer_emitted_count = 0;
-  // Below stat is not part of the spec.
+  // Below stats are not part of the spec.
   uint64_t voice_concealed_samples = 0;
   uint64_t delayed_packet_outage_samples = 0;
+  // This is sum of relative packet arrival delays of received packets so far.
+  // Since end-to-end delay of a packet is difficult to measure and is not
+  // necessarily useful for measuring jitter buffer performance, we report a
+  // relative packet arrival delay. The relative packet arrival delay of a
+  // packet is defined as the arrival delay compared to the first packet
+  // received, given that it had zero delay. To avoid clock drift, the "first"
+  // packet can be made dynamic.
+  uint64_t relative_packet_arrival_delay_ms = 0;
+  uint64_t jitter_buffer_packets_received = 0;
 };
 
 // Metrics that describe the operations performed in NetEq, and the internal