api/introspection: deprecation warnings in server

The Server rpc in introspection service is extended to expose
deprecation warnings based on observed feature use in containerd.

Signed-off-by: Samuel Karp <samuelkarp@google.com>

api/next.pb.txt

@@ -4344,6 +4344,7 @@ file {
   dependency: "github.com/containerd/containerd/api/types/platform.proto"
   dependency: "google/rpc/status.proto"
   dependency: "google/protobuf/empty.proto"
+  dependency: "google/protobuf/timestamp.proto"
   message_type {
     name: "Plugin"
     field {
@@ -4463,6 +4464,39 @@ file {
       type: TYPE_UINT64
       json_name: "pidns"
     }
+    field {
+      name: "deprecations"
+      number: 4
+      label: LABEL_REPEATED
+      type: TYPE_MESSAGE
+      type_name: ".containerd.services.introspection.v1.DeprecationWarning"
+      json_name: "deprecations"
+    }
+  }
+  message_type {
+    name: "DeprecationWarning"
+    field {
+      name: "id"
+      number: 1
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "id"
+    }
+    field {
+      name: "message"
+      number: 2
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "message"
+    }
+    field {
+      name: "last_occurrence"
+      number: 3
+      label: LABEL_OPTIONAL
+      type: TYPE_MESSAGE
+      type_name: ".google.protobuf.Timestamp"
+      json_name: "lastOccurrence"
+    }
   }
   service {
     name: "Introspection"