Fix logging integrations for DroppedSpan spans

[basepi]

The logging integrations assumed (erroneously) that all spans would have an ID. This fixes that assumption, so that DroppedSpans don't cause exceptions.

I also fixed up some tests to use the elasticapm_client fixture instead of manually creating a Tracer object.

[beniwohli]

@basepi maybe it would make sense to add an id attribute to DroppedSpan that is always None. That would make it less of a cognitive overhead to always have to remember that there are multiple types of spans. WDYT?

[basepi]

Seems reasonable to me. I'll make that change today.

[basepi]

@beniwohli Ready for review. Note that I do not include None IDs in the structlog integration. I'm a little unsure on that decision -- in most cases None would mean there isn't a transaction/span/trace but in this case we do have a span but it doesn't have an ID...

Probably not a terribly important distinction but I wanted to call it out explicitly in case you think I should include the field in this case.

[beniwohli]

other than the __slots__ thing, LGTM!