ext_proc: Implement message timeout (#15339)

Since ext_proc is a request/response protocol on a gRPC stream, a server may fail to respond in a timely way. The "message_timeout" field in the configuration controls how long the proxy will wait for an external processing server to respond to an individual message before giving up and returning an error (or not depending on configuration).

Signed-off-by: Gregory Brail <gregbrail@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 5204a63a49ff09fcd57f189c2addf7f36ffac87c

envoy/extensions/filters/http/ext_proc/v3alpha/ext_proc.proto

@@ -118,13 +118,13 @@ message ExternalProcessor {
   // for the list of supported attributes and their types.
   repeated string response_attributes = 6;
 
-  // [#not-implemented-hide:]
   // Specifies the timeout for each individual message sent on the stream and
   // when the filter is running in synchronous mode. Whenever
   // the proxy sends a message on the stream that requires a response, it will
   // reset this timer, and will stop processing and return an error (subject
-  // to the processing mode) if the timer expires. There is no timeout when
-  // the filter is running in asynchronous mode. Default is 200 ms.
+  // to the processing mode) if the timer expires before a matching response.
+  // is received. There is no timeout when the filter is running in asynchronous
+  // mode. Default is 200 milliseconds.
   google.protobuf.Duration message_timeout = 7;
 
   // [#not-implemented-hide:]