Implement a way to set span attributes without overriding

[unflxw]

This issue is spun off from a comment in #862. Implement a way to set the name and attributes for a span without overriding it, similar to how it was done for the sample data in #862.

To do

• Implement and expose set_attribute_if_nil
• Implement and expose set_name_if_nil, Add set_name_if_nil helper #944
• Use them in our instrumentations if/when relevant (at close time)

[tombruijn]

I've added it for span name, but won't implement it for attributes, because I have no purpose for it myself now. Unassigning and throwing it back on the backlog.