Add Lazy attributes for Events

[GoPavel]

Specification of Event states that Lazy Event is

lazily constructed, with the intention of avoiding unnecessary work if an event is unused

But now it's not true. In resolved PR#130 mentions lazy attributes for Event and Link in the future, but I don't see an issue about it.

Current code of Span.add_lazy_event and Span.add_event in SDK looks strange.

[mauriciovasquezbernal]

Hi @GoPavel,

You're right, the current implementation is wrong and it doesn't support lazy events.
A possible implementation could be to add Callable as an attribute type, exporters should perform a type check and invoke the attribute if it is a Callable to get its value.

[Oberon00]

Does the same apply to "lazy" links? That also always looked strange to me but I never really understood the purpose of lazy links tbh.

[mauriciovasquezbernal]

Yes, lazy links were also wrongly implemented by me, somehow I misunderstood that lazy meant to receive an Event object instead of receiving the parameters and constructing the object internally.

Btw, lazy links don't exist anymore #259.

I think we can provide real support for lazy events using Callable in Python.