upstream: set response flag on stream idle timeout (#5869)

This adds a new response flag SI that is set when the request is
canceled due to a stream idle timeout.

_Risk Level_: Low
_Testing_: Updated existing access log UTs, added test for setting flag
_Docs Changes_: Updated list of response flags
_Release Notes_: Added release note

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ a22d983cc87170677243d4aeba6457a4972d7980

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

@@ -187,7 +187,8 @@ message ResponseFlagFilter {
       "UAEX",
       "RLSE",
       "DC",
-      "URX"
+      "URX",
+      "SI"
     ]
   }];
 }

envoy/data/accesslog/v2/accesslog.proto

@@ -197,6 +197,9 @@ message ResponseFlags {
 
   // Indicates that the upstream retry limit was exceeded, resulting in a downstream error.
   bool upstream_retry_limit_exceeded = 16;
+
+  // Indicates that the stream idle timeout was hit, resulting in a downstream 408.
+  bool stream_idle_timeout = 17;
 }
 
 // [#not-implemented-hide:]