udp proxy: complete MVP (#9046)

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

Mirrored from https://github.com/envoyproxy/envoy @ aa3cb57d7cbe7db361c7bd8c08fadfd45d1dda30

docs/BUILD

@@ -66,6 +66,7 @@ proto_library(
         "//envoy/config/filter/network/zookeeper_proxy/v1alpha1:pkg",
         "//envoy/config/filter/thrift/rate_limit/v2alpha1:pkg",
         "//envoy/config/filter/thrift/router/v2alpha1:pkg",
+        "//envoy/config/filter/udp/udp_proxy/v2alpha:pkg",
         "//envoy/config/grpc_credential/v2alpha:pkg",
         "//envoy/config/health_checker/redis/v2:pkg",
         "//envoy/config/listener/v2:pkg",

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

@@ -10,7 +10,9 @@ import "google/protobuf/duration.proto";
 
 import "validate/validate.proto";
 
-// TODO(mattklein123): docs
+// [#protodoc-title: UDP proxy]
+// UDP proxy :ref:`configuration overview <config_udp_listener_filters_udp_proxy>`.
+// [#extension: envoy.filters.udp_listener.udp_proxy]
 
 // Configuration for the UDP proxy filter.
 message UdpProxyConfig {