Document user-defined metadata for swift samples

Add a brief documentation on how to configure and use
user-defined metadata for swift samples based on
'metadata_headers' property in swift middleware.

Change-Id: I365db3fda74a6e6ef0e8bcfbad917bcdac8e02ee
Revert "Support building wheels (PEP-427)"

Monty wrote:

  This is incorrect. universal indicates that a created wheel works with
  both py2 and py3. We can build wheels without this. Infra has patches
  almost landed to create and upload wheels on every release

This reverts commit db4b7fa36cc9270c6d272070de7bc77f630821ca.

Change-Id: I67ce638ce1237f7dda5dc041dc83544bced47669
Merge "Document user-defined metadata for swift samples"
Merge "Revert "Support building wheels (PEP-427)""