Prototype common attributes

[martinkuba]

This will include attributes described in this PR - session ID, URL, page width and height etc.

The main goal is to prototype the mechanism for how these attributes are captured and exported. Per discussions with @jack-berg and @jsuereth, the attributes should be added to individual spans/logs (instead of resource attributes). In this prototype, the attributes should be added using a span/log processor. The processor will retrieve the attributes from some central place and add them to every span/log.

The mechanism for adding/retrieving these attributes needs to be defined. One possible option is to store them in Context.