Fail span if error tag is true

[jizhang27]

Context

From opentracing spec, error tag should be respected to "decide if and only if the application considers the operation represented by the Span to have failed". true if and only if the application considers the operation represented by the Span to have failed

Currently we are relying on those three tags instead: ['error.type', 'error.msg', 'error.stack'].
reference

Approach

Set error flag to 1 if error tag is true. Also, skip writing it to meta since the error flag (not the tag) is used to classify the error.

TODO

• Squash the commits if necessary

Closes #139

[rochdev]

Thanks for the PR!

However, the statement "decide if and only if the application considers the operation represented by the Span to have failed" is incorrect.

The exact wording from the specification for the error tag is "true if and only if the application considers the operation represented by the Span to have failed". This simply means that the error tag must not be set to true if the Span hasn't failed.

Since the backend relies on the error field and not the error tag, it should only be used by users who want to use the OpenTracing API.

The following should happen in the formatter when the error tag is set to true:

• Set the error field to 1 on the Span since this is the value that is actually used by the backend to classify errors.
• Discard the error tag since it' redundant after setting the error field and has no added value at this point.

For other use cases the current behavior is correct.

[jizhang27]

Thanks for the comments @rochdev!

please correct me if i get it wrong, what opentracing only specifies the value of error tag itself instead of the error field used by the backed.

In this case, will my first commit be good enough then?

[rochdev]

Yes, the first commit would indeed be the correct way to solve this issue.

[jizhang27]

comments addressed with description updated.

[rochdev]

LGTM, will be in next release. Thanks @jizhang27!