upstream: Optimize subset_lb for case of a single host per subset (#12594)

This can be used in similar situations to maglev/ring, but can be used when the host selection must remain the same for existing sessions even if more hosts are added to the cluster.

In a test with 1500 endpoints, updating 50 (remove 50, add 50 different ones) took:
without new setting: ~500ms
with new setting (this PR): ~4ms

Signed-off-by: Greg Greenway <ggreenway@apple.com>

Mirrored from https://github.com/envoyproxy/envoy @ 724df08c329c8270509e2a8568be90fd43d597c1

envoy/config/cluster/v3/cluster.proto

@@ -246,6 +246,20 @@ message Cluster {
       // List of keys to match with the weighted cluster metadata.
       repeated string keys = 1;
 
+      // Selects a mode of operation in which each subset has only one host. This mode uses the same rules for
+      // choosing a host, but updating hosts is faster, especially for large numbers of hosts.
+      //
+      // If a match is found to a host, that host will be used regardless of priority levels, unless the host is unhealthy.
+      //
+      // Currently, this mode is only supported if `subset_selectors` has only one entry, and `keys` contains
+      // only one entry.
+      //
+      // When this mode is enabled, configurations that contain more than one host with the same metadata value for the single key in `keys`
+      // will use only one of the hosts with the given key; no requests will be routed to the others. The cluster gauge
+      // :ref:`lb_subsets_single_host_per_subset_duplicate<config_cluster_manager_cluster_stats_subset_lb>` indicates how many duplicates are
+      // present in the current configuration.
+      bool single_host_per_subset = 4;
+
       // The behavior used when no endpoint subset matches the selected route's
       // metadata.
       LbSubsetSelectorFallbackPolicy fallback_policy = 2

envoy/config/cluster/v4alpha/cluster.proto

@@ -249,6 +249,20 @@ message Cluster {
       // List of keys to match with the weighted cluster metadata.
       repeated string keys = 1;
 
+      // Selects a mode of operation in which each subset has only one host. This mode uses the same rules for
+      // choosing a host, but updating hosts is faster, especially for large numbers of hosts.
+      //
+      // If a match is found to a host, that host will be used regardless of priority levels, unless the host is unhealthy.
+      //
+      // Currently, this mode is only supported if `subset_selectors` has only one entry, and `keys` contains
+      // only one entry.
+      //
+      // When this mode is enabled, configurations that contain more than one host with the same metadata value for the single key in `keys`
+      // will use only one of the hosts with the given key; no requests will be routed to the others. The cluster gauge
+      // :ref:`lb_subsets_single_host_per_subset_duplicate<config_cluster_manager_cluster_stats_subset_lb>` indicates how many duplicates are
+      // present in the current configuration.
+      bool single_host_per_subset = 4;
+
       // The behavior used when no endpoint subset matches the selected route's
       // metadata.
       LbSubsetSelectorFallbackPolicy fallback_policy = 2