http: Add HCM log on successful tunnel establishment. (#27292)

* Add HCM log on successful tunnel establishment.

Signed-off-by: Paul Gallagher <pgal@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 491269b6f6ac1dc70711c47d7004f135d8711614

envoy/data/accesslog/v3/accesslog.proto

@@ -43,6 +43,7 @@ enum AccessLogType {
   UpstreamPoolReady = 7;
   UpstreamPeriodic = 8;
   UpstreamEnd = 9;
+  DownstreamTunnelSuccessfullyEstablished = 10;
 }
 
 message TCPAccessLogEntry {

envoy/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto

@@ -372,6 +372,11 @@ message HttpConnectionManager {
     // This log record, if enabled, does not depend on periodic log records or request completion log.
     // Details related to upstream cluster, such as upstream host, will not be available for this log.
     bool flush_access_log_on_new_request = 2;
+
+    // If true, the HCM will flush an access log when a tunnel is successfully established. For example,
+    // this could be when an upstream has successfully returned 101 Switching Protocols, or when the proxy
+    // has returned 200 to a CONNECT request.
+    bool flush_log_on_tunnel_successfully_established = 3;
   }
 
   reserved 27, 11;