Fix missing sampler tags in generated root span when startSpan

[ChenX1993]

Signed-off-by: Chen Xu chen.x@uber.com

Which problem is this PR solving?

• Resolves #611

Short description of the changes

• Remove the code to create a new slice to sp.tags
• Update and add unit tests to verify

Additional explanation

I understand the jaeger-client-go is deprecated, and no new PR request is accepted. I just need to create a public PR (not need to merge), then I can patch in our own internal repo following our process.