Add ExtProcClientInterceptor as the last in chain since it erases the type information for downstream interceptors

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/XdsNameResolver.java

@@ -817,6 +817,7 @@ private ClientInterceptor createFilters(
       }
 
       ImmutableList.Builder<ClientInterceptor> filterInterceptors = ImmutableList.builder();
+      ClientInterceptor extProcInterceptor = null;
       for (NamedFilterConfig namedFilter : filterConfigs) {
         String name = namedFilter.name;
         FilterConfig config = namedFilter.filterConfig;
@@ -829,10 +830,18 @@ private ClientInterceptor createFilters(
             filter.buildClientInterceptor(config, overrideConfig, scheduler);
 
         if (interceptor != null) {
-          filterInterceptors.add(interceptor);
+          if (config.typeUrl().equals(ExternalProcessorFilter.TYPE_URL)) {
+            extProcInterceptor = interceptor;
+          } else {
+            filterInterceptors.add(interceptor);
+          }
         }
       }
 
+      if (extProcInterceptor != null) {
+        filterInterceptors.add(extProcInterceptor);
+      }
+
       // Combine interceptors produced by different filters into a single one that executes
       // them sequentially. The order is preserved.
       return combineInterceptors(filterInterceptors.build());