[envoy/router] Move MetadataMatchCriteriaImpl implementation to its own source file (#4406)

Making this change so others can depend on metadatamatchcriteria_impl without needing
config_impl. Also update comments on metadata_match to specify that these fields only apply for the subset load balancer.

Context: https://github.com/envoyproxy/envoy/pull/4402/files#r216856765

Risk Level: low
Testing: tests, new and old, pass
Doc Changes: clarified docs
Release Notes: n/a

Signed-off-by: Brian Ramos <brirams@users.noreply.github.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3b3c28a1dd6be6e1dd11e543227b23891c234fe8

WORKSPACE

@@ -1,49 +0,0 @@
-workspace(name = "envoy_api")
-
-load("//bazel:repositories.bzl", "api_dependencies")
-
-api_dependencies()
-
-http_archive(
-    name = "com_google_protobuf",
-    sha256 = "0cc6607e2daa675101e9b7398a436f09167dffb8ca0489b0307ff7260498c13c",
-    strip_prefix = "protobuf-3.5.0",
-    urls = ["https://github.com/google/protobuf/archive/v3.5.0.tar.gz"],
-)
-
-http_archive(
-    name = "com_google_protobuf_cc",
-    sha256 = "0cc6607e2daa675101e9b7398a436f09167dffb8ca0489b0307ff7260498c13c",
-    strip_prefix = "protobuf-3.5.0",
-    urls = ["https://github.com/google/protobuf/archive/v3.5.0.tar.gz"],
-)
-
-# Needed for ``bazel fetch`` to work with @com_google_protobuf
-# https://github.com/google/protobuf/blob/v3.5.0/util/python/BUILD#L6-L9
-bind(
-    name = "python_headers",
-    actual = "@com_google_protobuf//util/python:python_headers",
-)
-
-bind(
-    name = "six",
-    actual = "@six_archive//:six",
-)
-
-new_http_archive(
-    name = "six_archive",
-    build_file = "@com_google_protobuf//:six.BUILD",
-    sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
-    url = "https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz#md5=34eed507548117b2ab523ab14b2f8b55",
-)
-
-http_archive(
-    name = "io_bazel_rules_go",
-    url = "https://github.com/bazelbuild/rules_go/releases/download/0.8.1/rules_go-0.8.1.tar.gz",
-    sha256 = "90bb270d0a92ed5c83558b2797346917c46547f6f7103e648941ecdb6b9d0e72",
-)
-load("@io_bazel_rules_go//go:def.bzl", "go_rules_dependencies", "go_register_toolchains")
-load("@com_lyft_protoc_gen_validate//bazel:go_proto_library.bzl", "go_proto_repositories")
-go_proto_repositories(shared=0)
-go_rules_dependencies()
-go_register_toolchains()

envoy/api/v2/route/route.proto

@@ -188,9 +188,11 @@ message WeightedCluster {
     // entries in the clusters array must add up to the total_weight, which defaults to 100.
     google.protobuf.UInt32Value weight = 2;
 
-    // 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*.
+    // Optional endpoint metadata match criteria used by the subset load balancer. Only endpoints in
+    // the upstream cluster with metadata matching what is set in this field will be considered for
+    // load balancing. Note that this will be merged with what's provided in :ref:
+    // `RouteAction.MetadataMatch <envoy_api_field_route.RouteAction.metadata_match>`, with values
+    // here taking precedence. The filter name should be specified as *envoy.lb*.
     core.Metadata metadata_match = 3;
 
     // Specifies a list of headers to be added to requests when this cluster is selected
@@ -372,9 +374,11 @@ message RouteAction {
   ClusterNotFoundResponseCode cluster_not_found_response_code = 20
       [(validate.rules).enum.defined_only = true];
 
-  // 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*.
+  // Optional endpoint metadata match criteria used by the subset load balancer. Only endpoints
+  // in the upstream cluster with metadata matching what's set in this field will be considered
+  // for load balancing. If using :ref:`weighted_clusters
+  // <envoy_api_field_route.RouteAction.weighted_clusters>`, metadata will be merged, with values
+  // provided there taking precedence. The filter name should be specified as *envoy.lb*.
   core.Metadata metadata_match = 4;
 
   // Indicates that during forwarding, the matched prefix (or path) should be