[Bug]: Sampler tags are missing in root span generated from startSpan

[ChenX1993]

What happened?

The sampler tags (sampler.type & sampler.param) are missing in generated root span when additional tags option is provided in startSpan.

Steps to reproduce

1. call tracer.StartSpan with tag option that has more than 2 key-value pairs.

t := opentracing.Tags{
"testTag1": "test tag 1",
"testTag2": "test tag 2",
"testTag3": "test tag 3",
}
sp := tracer.StartSpan("operation _name", t)
ss := sp.(*Span)

Expected behavior

The span should have expected sampler.type & sampler.param tags.

Relevant log output

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Additional context

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Jaeger backend version

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