Make echo metrics optional

This makes it optional for an echo detector to report metrics through
the getStats interface.

Bug: webrtc:11539
Change-Id: I1fef93b7bf534637b69c16971d38709b3e849a08
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174100
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31146}
diff --git a/modules/audio_processing/include/audio_processing.h b/modules/audio_processing/include/audio_processing.h
index 9c2b09f..5f90d7d 100644
--- a/modules/audio_processing/include/audio_processing.h
+++ b/modules/audio_processing/include/audio_processing.h
@@ -884,8 +884,8 @@
                                     std::vector<float>* packed_buffer);
 
   struct Metrics {
-    double echo_likelihood;
-    double echo_likelihood_recent_max;
+    absl::optional<double> echo_likelihood;
+    absl::optional<double> echo_likelihood_recent_max;
   };
 
   // Collect current metrics from the echo detector.