Added generic mechanism to provide the ability to extract span attributes

[codeboten]

Multiple instrumentation packages are currently implementing the ability to extract additional attributes from requests via a configuration option, this should be refactored to be re-used in additional instrumentations.

#1158
#1154

[falau]

Hello, I just came across this topic and wondering if it's possible to support multilevel lookup on extracting attributes? Likerequest.attr1.attr2.
Currently all implementations mentioned use getattr and then convert those attributes with str(), which can only look up one level of attributes. However, my use case is to access both request.user(which is str(request.user.username) in Django) and another custom attribute under it such as request.user.myattr for later use.

How about using operator.attrgetter(attr) than a getattr here?