Add @wraps to the inner function of @ptransform_fn decorator

Due to the absence of the @wraps decorator on the inner function,
the metadata about the callable passed(fn.__name__) was not being
updated in the @ptransform_fn decorator.

sdks/python/apache_beam/transforms/ptransform.py

@@ -46,7 +46,7 @@ class and wrapper class that allows lambda functions to be used as
 from builtins import hex
 from builtins import object
 from builtins import zip
-from functools import reduce
+from functools import reduce, wraps
 
 from google.protobuf import message
 
@@ -854,7 +854,7 @@ def expand(self, pcoll):
   (first argument if no label was specified and second argument otherwise).
   """
   # TODO(robertwb): Consider removing staticmethod to allow for self parameter.
-
+  @wraps(fn)
   def callable_ptransform_factory(*args, **kwargs):
     return _PTransformFnPTransform(fn, *args, **kwargs)
   return callable_ptransform_factory