Support Envoy Native Subset Load Balancing[WIP]

networking/v1alpha3/destination_rule.proto

@@ -518,6 +518,8 @@ message LoadBalancerSettings {
   // Locality load balancer settings, this will override mesh wide settings in entirety, meaning no merging would be performed
   // between this object and the object one in MeshConfig
   LocalityLoadBalancerSetting locality_lb_setting = 3;
+
+  LoadBalancerSubsetSetting lb_subset_setting = 4;
 }
 
 // Connection pool settings for an upstream host. The settings apply to
@@ -1069,3 +1071,54 @@ message LocalityLoadBalancerSetting{
     // e.g. true means that turn on locality load balancing for this DestinationRule no matter what mesh wide settings is.
     google.protobuf.BoolValue enabled = 3;
 }
+
+// Locality load balancing settings.
+message LoadBalancerSubsetSetting {
+
+  // If NO_FALLBACK is selected, a result
+  // equivalent to no healthy hosts is reported. If ANY_ENDPOINT is selected,
+  // any cluster endpoint may be returned (subject to policy, health checks,
+  // etc). If DEFAULT_SUBSET is selected, load balancing is performed over the
+  // endpoints matching the values from the default_subset field.
+  enum LbSubsetFallbackPolicy {
+    NO_FALLBACK = 0;
+    ANY_ENDPOINT = 1;
+    DEFAULT_SUBSET = 2;
+  };
+
+  LbSubsetFallbackPolicy fallback_policy = 1;
+
+  // Metadata on endpoint 
+  map<string, string> metadata = 2;
+
+  // Specifications for subsets.
+  message SubsetSelector{
+    // Allows to override top level fallback policy per selector.
+    enum SubsetSelectorFallbackPolicy {
+      // If NOT_DEFINED top level config fallback policy is used instead.
+      NOT_DEFINED = 0;
+
+      // If NO_FALLBACK is selected, a result equivalent to no healthy hosts is reported.
+      NO_FALLBACK = 1;
+
+      // If ANY_ENDPOINT is selected, any cluster endpoint may be returned
+      // (subject to policy, health checks, etc).
+      ANY_ENDPOINT = 2;
+
+      // If DEFAULT_SUBSET is selected, load balancing is performed over the
+      // endpoints matching the values from the default_subset field.
+      DEFAULT_SUBSET = 3;
+    }
+
+    // List of keys to match with the weighted cluster metadata.
+    repeated string keys = 1;
+
+    // The behavior used when no endpoint subset matches the selected route's
+    // metadata.
+    SubsetSelectorFallbackPolicy fallback_policy = 2;
+  };
+
+  // One or more subset selectors
+  repeated SubsetSelector subset_selectors = 3;
+}
+