udp_proxy: implement idle timeout and some stats (#8999)

Another bunch of work towards
https://github.com/envoyproxy/envoy/issues/492.

The remaining work is proper wiring up of upstream cluster
management, host health, etc. and documentation. This will
be done in the next PR.

Signed-off-by: Matt Klein <mklein@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ 647c1eeba8622bafdd6add1e7997c1f0bda31be5

envoy/config/filter/udp/udp_proxy/v2alpha/udp_proxy.proto

@@ -12,11 +12,19 @@ import "validate/validate.proto";
 
 // TODO(mattklein123): docs
 
+// Configuration for the UDP proxy filter.
 message UdpProxyConfig {
-  oneof cluster_specifier {
+  // The stat prefix used when emitting UDP proxy filter stats.
+  string stat_prefix = 1 [(validate.rules).string = {min_bytes: 1}];
+
+  oneof route_specifier {
     option (validate.required) = true;
 
     // The upstream cluster to connect to.
-    string cluster = 1 [(validate.rules).string = {min_bytes: 1}];
+    string cluster = 2 [(validate.rules).string = {min_bytes: 1}];
   }
+
+  // The idle timeout for sessions. Idle is defined as no datagrams between received or sent by
+  // the session. The default if not specified is 1 minute.
+  google.protobuf.Duration idle_timeout = 3;
 }