access_logs: resolve current duration for %DURATION% (#26203)

Signed-off-by: Ohad Vano <ohadvano@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3a56e84b4f4b38ed6278f66c8411f038d697114c

envoy/config/accesslog/v3/accesslog.proto

@@ -122,7 +122,10 @@ message StatusCodeFilter {
   ComparisonFilter comparison = 1 [(validate.rules).message = {required: true}];
 }
 
-// Filters on total request duration in milliseconds.
+// Filters based on the duration of the request or stream, in milliseconds.
+// For end of stream access logs, the total duration of the stream will be used.
+// For :ref:`periodic access logs<arch_overview_access_log_periodic>`,
+// the duration of the stream at the time of log recording will be used.
 message DurationFilter {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.filter.accesslog.v2.DurationFilter";