ext/Zipkin - Transform resource to tags when exporting

[nirsky]

This PR implements the missing part of exporting the TraceProvider resource into Zipkin.
Same as in js.

This PR also aligns Zipkin behavior with my previous PR done to jaeger - #645.

I wanted to preserve the existing behavior of the tags being None if attributes do not exist, so I ended with a nested if, although I think it's worth discussing maybe the default value should always be {}.

As for the test, I added another span to cover all tags permutations:

1. Span only has attributes.
2. Span only has resource.
3. Span has both resource and attributes.
4. Span doesn't have either.

[toumorokoshi]

Thanks for the PR! I think there's a few code improvements that can be made here, see comments.

[toumorokoshi]

there are cases where a DefaultSpan is used instead of an sdk.trace.Span. In those cases this will return an attributeerror as "attributes" doesn't exist.

There may need to be some sort of getter for attributes in the API itself to ensure this works as expected.

[nirsky]

Can you elaborate on how I should handle this?
Currently I kept the existing behavior.

[codeboten]

Right, this was a bug that existed in the current implementation of the exporter. You'd want to use something like getattr(span, "attributes", None) to prevent an exception from being raised.

[nirsky]

Thanks for clarifying! Changed to getattr.

[codeboten]

Thanks for adding this, looking at the code, there are a few places that are making the assumption that a span has an attributes attribute, I've created the issue here to track the work: #715

[codeboten]

Right, this was a bug that existed in the current implementation of the exporter. You'd want to use something like getattr(span, "attributes", None) to prevent an exception from being raised.

[toumorokoshi]

LGTM! Thanks for addressing my concerns.

[toumorokoshi]

Thanks!