access log: identify downstream disconnects in HCM (#5229)

This identifies downstream disconnects from within the HTTP Connection
Manager and eventually echoes it out via the access log.

Signed-off-by: Venil Noronha <veniln@vmware.com>

Mirrored from https://github.com/envoyproxy/envoy @ 988f7d81f973e17c76db3f900cf145c371d6e8b3

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

@@ -168,6 +168,22 @@ message ResponseFlagFilter {
   // This field is optional. If it is not specified, then any response flag will pass
   // the filter check.
   repeated string flags = 1 [(validate.rules).repeated .items.string = {
-    in: ["LH", "UH", "UT", "LR", "UR", "UF", "UC", "UO", "NR", "DI", "FI", "RL", "UAEX", "RLSE"]
+    in: [
+      "LH",
+      "UH",
+      "UT",
+      "LR",
+      "UR",
+      "UF",
+      "UC",
+      "UO",
+      "NR",
+      "DI",
+      "FI",
+      "RL",
+      "UAEX",
+      "RLSE",
+      "DC"
+    ]
   }];
 }

envoy/data/accesslog/v2/accesslog.proto

@@ -153,7 +153,7 @@ message ResponseFlags {
   // Indicates there was a local reset by a connection pool due to an initial connection failure.
   bool upstream_connection_failure = 6;
 
-  // Indicates the stream was reset locally due to connection termination.
+  // Indicates the stream was reset due to an upstream connection termination.
   bool upstream_connection_termination = 7;
 
   // Indicates the stream was reset because of a resource overflow.
@@ -188,6 +188,9 @@ message ResponseFlags {
   // [#not-implemented-hide:] Hide from docs.
   // Indicates that the request was rejected because there was an error in rate limit service.
   bool rate_limit_service_error = 14;
+
+  // Indicates the stream was reset due to a downstream connection termination.
+  bool downstream_connection_termination = 15;
 }
 
 // [#not-implemented-hide:]