modified condition for trailing metadata.
added more information to log message.

changed core logic for executing multiple ops.
added call cancellation logic
simplified the code.

gpr_slice_new_with_user_data is like gpr_slice_new, but allows the
caller to specify a distinct pointer to pass to the destroy function.
This is useful when the data is part of a larger data structure that
should be destroyed when the data is no longer needed.