Introduce debug network stats

Bug: webrtc:11959
Change-Id: I29e94cf1cdc9aee2bbe4396aa14a759c1a9ae560
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/184600
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32170}

api/numerics/samples_stats_counter.h

@@ -45,6 +45,8 @@ class SamplesStatsCounter {
 
   // Returns if there are any values in O(1) time.
   bool IsEmpty() const { return samples_.empty(); }
+  // Returns the amount of samples added into counter in O(1) time.
+  int64_t NumSamples() const { return stats_.Size(); }
 
   // Returns min in O(1) time. This function may not be called if there are no
   // samples.