tcp proxy: add metadata_match to tcp proxy config (#520)

* tcp proxy: add metadata_match to tcp proxy config

This should allow the tcp proxy to target a specific subset of endpoints
in a cluster by matching on the metadata, similar to what is currently
possible in the http_connection_manager filter.

See https://github.com/envoyproxy/envoy/issues/2696

Signed-off-by: Snow Pettersen <snowp@squareup.com>

envoy/config/filter/network/tcp_proxy/v2/BUILD

@@ -7,6 +7,7 @@ api_proto_library(
     srcs = ["tcp_proxy.proto"],
     deps = [
         "//envoy/api/v2/core:address",
+        "//envoy/api/v2/core:base",
         "//envoy/config/filter/accesslog/v2:accesslog",
     ],
 )

envoy/config/filter/network/tcp_proxy/v2/tcp_proxy.proto

@@ -5,6 +5,7 @@ option go_package = "v2";
 
 import "envoy/config/filter/accesslog/v2/accesslog.proto";
 import "envoy/api/v2/core/address.proto";
+import "envoy/api/v2/core/base.proto";
 
 import "google/protobuf/duration.proto";
 import "google/protobuf/wrappers.proto";
@@ -34,6 +35,12 @@ message TcpProxy {
   //
   string cluster = 2;
 
+  // [#not-implemented-hide:]
+  // Optional endpoint metadata match criteria. Only endpoints in the upstream
+  // cluster with metadata matching that set in metadata_match will be
+  // considered. The filter name should be specified as *envoy.lb*.
+  envoy.api.v2.core.Metadata metadata_match = 9;
+
   // The idle timeout for connections managed by the TCP proxy filter. The idle timeout
   // is defined as the period in which there are no bytes sent or received on either
   // the upstream or downstream connection. If not set, connections will never be closed