Add overload manager for Envoy (#3954)

Signed-off-by: Elisha Ziskind <eziskind@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 83b9e2da8ad896f6e2a0b54e08cc79a6937714c8

envoy/config/overload/v2alpha/BUILD

@@ -0,0 +1,8 @@
+load("//bazel:api_build_system.bzl", "api_proto_library_internal")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_library_internal(
+    name = "overload",
+    srcs = ["overload.proto"],
+)

envoy/config/overload/v2alpha/overload.proto

@@ -0,0 +1,63 @@
+syntax = "proto3";
+
+package envoy.config.overload.v2alpha;
+option go_package = "v2alpha";
+
+import "google/protobuf/duration.proto";
+import "google/protobuf/struct.proto";
+
+import "validate/validate.proto";
+
+// The Overload Manager provides an extensible framework to protect Envoy instances
+// from overload of various resources (memory, cpu, file descriptors, etc)
+
+message EmptyConfig {
+}
+
+message ResourceMonitor {
+  // The name of the resource monitor to instantiate. Must match a registered
+  // resource monitor type.
+  string name = 1 [(validate.rules).string.min_bytes = 1];
+
+  // Configuration for the resource monitor being instantiated.
+  google.protobuf.Struct config = 2;
+}
+
+message ThresholdTrigger {
+  // If the resource pressure is greater than or equal to this value, the trigger
+  // will fire.
+  double value = 1 [(validate.rules).double = {gte: 0, lte: 1}];
+}
+
+message Trigger {
+  // The name of the resource this is a trigger for.
+  string name = 1 [(validate.rules).string.min_bytes = 1];
+
+  oneof trigger_oneof {
+    option (validate.required) = true;
+    ThresholdTrigger threshold = 2;
+  }
+}
+
+message OverloadAction {
+  // The name of the overload action. This is just a well-known string that listeners can
+  // use for registering callbacks. Custom overload actions should be named using reverse
+  // DNS to ensure uniqueness.
+  string name = 1 [(validate.rules).string.min_bytes = 1];
+
+  // A set of triggers for this action. If any of these triggers fires the overload action
+  // is activated. Listeners are notified when the overload action transitions from
+  // inactivated to activated, or vice versa.
+  repeated Trigger triggers = 2 [(validate.rules).repeated .min_items = 1];
+}
+
+message OverloadManager {
+  // The interval for refreshing resource usage.
+  google.protobuf.Duration refresh_interval = 1;
+
+  // The set of resources to monitor.
+  repeated ResourceMonitor resource_monitors = 2 [(validate.rules).repeated .min_items = 1];
+
+  // The set of overload actions.
+  repeated OverloadAction actions = 3 [(validate.rules).repeated .min_items = 1];
+}