Add a Metadata protobuf module

Cargo.toml

@@ -18,6 +18,7 @@ destination = ["http-types", "net", "prost-types", "tonic/codegen"]
 http-types = ["http", "thiserror"]
 identity = ["prost-types", "tonic/codegen"]
 inbound = ["net", "prost-types", "tonic/codegen"]
+meta = []
 net = ["ipnet", "thiserror"]
 tap = ["h2", "http-types", "net", "prost-types", "tonic/codegen"]
 

proto/inbound.proto

@@ -5,6 +5,7 @@ package io.linkerd.proxy.inbound;
 option go_package = "github.com/linkerd/linkerd2-proxy-api/go/inbound";
 
 import "google/protobuf/duration.proto";
+import "meta.proto";
 import "net.proto";
 
 /// An API exposed to the linkerd2-proxy to configure the inbound proxy with per-port configuration
@@ -108,7 +109,15 @@ message Authz {
   //
   // A control plane SHOULD return the same keys in all authorizations. That is,
   // we do NOT want to return arbitrary pod labels in this field.
+  //
+  // `labels` should be considered deprecated. `metadata` is preferred. However,
+  // controllers should continue to set `labels` for compatibility with older
+  // proxies.
   map<string, string> labels = 3;
+
+  // If set, describes an Authorization configuration. Replaces the free-from
+  // `labels` field.
+  io.linkerd.proxy.meta.Metadata metadata = 4;
 }
 
 // Describes a network of authorized clients.

proto/meta.proto

@@ -0,0 +1,27 @@
+syntax = "proto3";
+
+package io.linkerd.proxy.meta;
+
+option go_package = "github.com/linkerd/linkerd2-proxy-api/go/meta";
+
+// General metadata about a configuration object. Typically references either an
+// implicit default configuration or a Kubernetes resource.
+message Metadata {
+  oneof kind {
+    // A name describing a default/implicit configuration.
+    //
+    // For example, a policy default name like `all-authenticated` describes an
+    // implicit controller-implemented policy that does not exist as a cluster
+    // resource.
+    string default = 1;
+
+    Resource resource = 2;
+  }
+}
+
+// References a (e.g., Kubernetes) resource.
+message Resource {
+  string group = 1;
+  string kind = 2;
+  string name = 3;
+}