Access log trailers (#3187)

Signed-off-by: Alexey Baranov <me@kotiki.cc>

Mirrored from https://github.com/envoyproxy/envoy @ ead0de979c339f2dfb209ebdd1c93ee1ef7c7d74

envoy/config/accesslog/v2/als.proto

@@ -27,6 +27,9 @@ message HttpGrpcAccessLogConfig {
 
   // Additional response headers to log in *HTTPResponseProperties.response_headers*.
   repeated string additional_response_headers_to_log = 3;
+
+  // Additional response trailers to log in *HTTPResponseProperties.response_trailers*.
+  repeated string additional_response_trailers_to_log = 4;
 }
 
 // Common configuration for gRPC access logs.

envoy/config/filter/accesslog/v2/accesslog.proto

@@ -269,6 +269,9 @@ message HTTPResponseProperties {
 
   // Map of additional headers configured to be logged.
   map<string, string> response_headers = 4;
+
+  // Map of trailers configured to be logged.
+  map<string, string> response_trailers = 5;
 }
 
 // [#not-implemented-hide:] Not configuration. TBD how to doc proto APIs.