New WebRTC histogram for Audio Processing Module runtime settings

The new histogram is used to keep track of how frequently APM runtime
settings cannot be successfully enqueued.

Used in https://webrtc-review.googlesource.com/c/src/+/190144

Bug: webrtc:9138
Change-Id: Ib2e67e2287f3e9d642db9712ddd829a14c2bd7ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495180Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ale Bzk <alessiob@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824341}

tools/metrics/histograms/histograms_xml/web_rtc/histograms.xml

@@ -216,6 +216,16 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Audio.ApmRuntimeSettingCannotEnqueue" units="counts"
+    expires_after="2021-12-31">
+  <owner>alessiob@chromium.org</owner>
+  <owner>peah@chromium.org</owner>
+  <summary>
+    Counts occurrences of if WebRTC's Audio Processing Module cannot enqueue a
+    runtime setting.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Audio.AudioInterruptionMs" units="ms"
     expires_after="2021-01-03">
   <owner>hlundin@chromium.org</owner>