Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/api/webrtcsession.cc

Index: webrtc/api/webrtcsession.cc
diff --git a/webrtc/api/webrtcsession.cc b/webrtc/api/webrtcsession.cc
index 35a5fab4c6ec08850f13ce8a11fe015ae6e61609..f8a8f678364a1ee5c68abddf60f7acfca64c6874 100644
--- a/webrtc/api/webrtcsession.cc
+++ b/webrtc/api/webrtcsession.cc
@@ -493,6 +493,8 @@ WebRtcSession::WebRtcSession(
       this, &WebRtcSession::OnTransportControllerCandidatesGathered);
   transport_controller_->SignalCandidatesRemoved.connect(
       this, &WebRtcSession::OnTransportControllerCandidatesRemoved);
+  transport_controller_->SignalDtlsHandshakeError.connect(
+      this, &WebRtcSession::OnDtlsHandshakeError);
 }
 
 WebRtcSession::~WebRtcSession() {
@@ -2073,4 +2075,12 @@ const std::string WebRtcSession::GetTransportName(
   }
   return channel->transport_name();
 }
+
+void WebRtcSession::OnDtlsHandshakeError(rtc::SSLHandshakeError error) {
+  if (metrics_observer_) {
+    metrics_observer_->IncrementEnumCounter(
+        webrtc::kEnumCounterDtlsHandshakeError, static_cast<int>(error),
+        static_cast<int>(rtc::SSLHandshakeError::MAX_VALUE));
+  }
+}
 }  // namespace webrtc