Add AEC filter divergence metric to StatsCollector.

A new metric that tells how often the AEC linear filter diverges has been recently introduced, see
https://codereview.webrtc.org/1739993003/

This metric can reflect echo failure and ducking.

In this CL, we add a field in StatsCollector to receive this metric.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1866983002 .

Cr-Commit-Position: refs/heads/master@{#12282}
diff --git a/webrtc/api/statscollector.cc b/webrtc/api/statscollector.cc
index 0901fc6..38fb5fb 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -937,6 +937,9 @@
         report, stats.typing_noise_detected, stats.echo_return_loss,
         stats.echo_return_loss_enhancement, stats.echo_delay_median_ms,
         stats.aec_quality_min, stats.echo_delay_std_ms);
+
+    report->AddFloat(StatsReport::kStatsValueNameAecDivergentFilterFraction,
+                     stats.aec_divergent_filter_fraction);
   }
 }