subset: allow matching metadata criteria with any element in list (#7047)

This makes it possible to configure the subset LB to match metadata
match criterias with any of the values specified in a list value on an
endpoint. This allows endpoints to have multiple values for a given
metadata key.

To accomplish this the invariants of the subset trie construction
changed: a host can now be associated with multiple subsets for a set of
subset selectors. To support this the trie construction had to change to
traverse all possible paths for each host.

Fixes #6921

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 41ecbb3e4bd48b425483e7c3aae17509f2ef3a80

envoy/api/v2/cds.proto

@@ -428,6 +428,11 @@ message Cluster {
     // subset might become empty. With this option enabled, if that happens the LB will attempt
     // to select a host from the entire cluster.
     bool panic_mode_any = 6;
+
+    // If true, metadata specified for a metadata key will be matched against the corresponding
+    // endpoint metadata if the endpoint metadata matches the value exactly OR it is a list value
+    // and any of the elements in the list matches the criteria.
+    bool list_as_any = 7;
   }
 
   // Configuration for load balancing subsetting.