Update REST Interceptors to support reading metadata

[parthea]

It's currently not possible to read response metadata using REST interceptors. This feature is missing but was part of the original requirements.

metadata is supported in the pre_ methods but not the post_methods

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2

Lines 323 to 325 in c72411d

	{{ async_prefix }}def pre_{{ method.name|snake_case }}(self, request, metadata):
	logging.log(f"Received request: {request}")
	return request, metadata

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/_shared_macros.j2

Lines 328 to 330 in c72411d

	{{ async_prefix }}def post_{{ method.name|snake_case }}(self, response):
	logging.log(f"Received response: {response}")
	return response