fix:(go/core/tracing) doing enhancements for unit test in span data

[falonso81]

Description here... Help the reviewer by:

• linking to an issue that includes more details
• add unit test for [Go] missing tracing metadata #225

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[hugoaguirre]

Instead of doing t.Errorf(); return you can use t.Fatalf()

[falonso81]

I did the change, but we get rid of the t testing

[apascal07]

I'm not sure how much value ensureAllSpanDataFieldsArePresent is adding here. We're essentially testing whether the JSON marshaling logic is working -- this should be well tested by Go core. The existing test reads the same trace.json file and compares the wire formats of both. What we want additionally (to address the linked issue) is a test of specific trace span labels -- something that we expect to be consistent between runtimes. I would look at a freshly generated trace span from JS and make sure that we are applying the same labels in Go and add a test for that (e.g. like the issue mentions, genkit:metadata:subtype).