feat(spanner): add leader_options to InstanceConfig and default_leader to Database

protos/google/spanner/admin/database/v1/spanner_database_admin.proto

@@ -399,6 +399,14 @@ message Database {
   // sure to account for the time from the moment when the value is queried to
   // the moment when you initiate the recovery.
   google.protobuf.Timestamp earliest_version_time = 7 [(google.api.field_behavior) = OUTPUT_ONLY];
+
+  // Output only. The read-write region which contains the database's leader
+  // replicas.
+  //
+  // This is the same as the value of default_leader
+  // database option set using DatabaseAdmin.CreateDatabase or
+  // DatabaseAdmin.UpdateDatabaseDdl. If not explicitly set, this is empty.
+  string default_leader = 9 [(google.api.field_behavior) = OUTPUT_ONLY];
 }
 
 // The request for [ListDatabases][google.spanner.admin.database.v1.DatabaseAdmin.ListDatabases].

protos/google/spanner/admin/instance/v1/spanner_instance_admin.proto

@@ -318,6 +318,10 @@ message InstanceConfig {
   // The geographic placement of nodes in this instance configuration and their
   // replication properties.
   repeated ReplicaInfo replicas = 3;
+
+  // Allowed values of the “default_leader” schema option for databases in
+  // instances that use this instance configuration.
+  repeated string leader_options = 4;
 }
 
 // An isolated set of Cloud Spanner resources on which databases can be hosted.