Add dynamic metadata support for ORIGINAL_DST cluster to pick a host (#28054)

One can specify a MetadataKey with a path selector to pick up a host
from the dynamic metadata of the request or downstream. Selected
value can either be a string or a list with at least a single
element of string type. Request metadata is considered first.

Signed-off-by: Andrii Chabykin <chabster@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 10468b320421cb14d7911b4e6d139cc18780fb1a

envoy/config/cluster/v3/BUILD

@@ -9,6 +9,7 @@ api_proto_package(
         "//envoy/annotations:pkg",
         "//envoy/config/core/v3:pkg",
         "//envoy/config/endpoint/v3:pkg",
+        "//envoy/type/metadata/v3:pkg",
         "//envoy/type/v3:pkg",
         "@com_github_cncf_udpa//udpa/annotations:pkg",
         "@com_github_cncf_udpa//xds/core/v3:pkg",

envoy/config/cluster/v3/cluster.proto

@@ -13,6 +13,7 @@ import "envoy/config/core/v3/health_check.proto";
 import "envoy/config/core/v3/protocol.proto";
 import "envoy/config/core/v3/resolver.proto";
 import "envoy/config/endpoint/v3/endpoint.proto";
+import "envoy/type/metadata/v3/metadata.proto";
 import "envoy/type/v3/percent.proto";
 
 import "google/protobuf/any.proto";
@@ -551,6 +552,10 @@ message Cluster {
     // The port to override for the original dst address. This port
     // will take precedence over filter state and header override ports
     google.protobuf.UInt32Value upstream_port_override = 3 [(validate.rules).uint32 = {lte: 65535}];
+
+    // The dynamic metadata key to override destination address.
+    // First the request metadata is considered, then the connection one.
+    type.metadata.v3.MetadataKey metadata_key = 4;
   }
 
   // Common configuration for all load balancer implementations.