… in ASGI.

Next part of open-telemetry#1184

Details:

Add test cases for regular expression matching.
Add test cases for header sanitizing.
Add documentation for regular expression matching and header sanitation.
Various documentation cleanups and standardization.
Fix keys() in class ASGIGetter so it returns the HTTP header keys instead of a list of available request data. This makes it consistent with the WSGIGetter keys() method.
Make ASGIGetter.get() compare all keys in a case insensitive manner.
Move TestCustomHeaders to a separate file to avoid module length error.