feat: add bazel support for gapic metadata (#811)

Author: software-dov

rules_python_gapic/py_gapic.bzl

@@ -18,13 +18,20 @@ def py_gapic_library(
         name,
         srcs,
         grpc_service_config = None,
-        plugin_args = [],
-        opt_args = [],
+        plugin_args = None,
+        opt_args = None,
+        metadata = False,
         **kwargs):
     #    srcjar_target_name = "%s_srcjar" % name
     srcjar_target_name = name
     srcjar_output_suffix = ".srcjar"
 
+    plugin_args = plugin_args or []
+    opt_args = opt_args or []
+
+    if metadata:
+        plugin_args.append("metadata")
+
     file_args = {}
     if grpc_service_config:
         file_args[grpc_service_config] =  "retry-config"