DNS: officially (previously only unofficially) allowing null resolutions (#19588)

in #17645 there was a bunch of discussion around the DNS cache returning null addresses and how to handle it. After discussion on #19461 we agreed to keep sending null updates, but to fast-fail if no address was resolved.

Risk Level: Medium (data plane change)
Testing: updated integration tests, unit tests
Docs Changes: n/a
Release Notes: inline

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ 2efe480d6b0a1501674780cfd7c343ef332010a1

envoy/data/accesslog/v3/accesslog.proto

@@ -192,7 +192,7 @@ message AccessLogCommon {
 }
 
 // Flags indicating occurrences during request/response processing.
-// [#next-free-field: 27]
+// [#next-free-field: 28]
 message ResponseFlags {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.data.accesslog.v2.ResponseFlags";
@@ -290,6 +290,9 @@ message ResponseFlags {
 
   // Indicates overload manager terminated the request.
   bool overload_manager = 26;
+
+  // Indicates a DNS resolution failed.
+  bool dns_resolution_failure = 27;
 }
 
 // Properties of a negotiated TLS connection.