Add get data of span method

[yore-new]

return the data of span to allow accessing all info of a span.
just return the value of data instead a pointer to avoid changing by user.
in order to solve #1007 .

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[yore-new]

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[basvanbeek]

As explained in #1007 there is no need to provide access to the entire span data payload for correlation purposes. This is what SpanContext is for. The other problem is you return a shallow copy of span data which would allow for mutating for instance the Attributes map (and get some concurrent access nasties).
For now, I do not see a reason for allowing access to even a full copy of span data.

[rakyll]

Why do we need to give SetForceGenerateSpanData as an option to the user?

@bogdandrutu, do we have any requirements about immutability of the span data?

[sydnash]

As explained in #1007 there is no need to provide access to the entire span data payload for correlation purposes. This is what SpanContext is for. The other problem is you return a shallow copy of span data which would allow for mutating for instance the Attributes map (and get some concurrent access nasties).
For now, I do not see a reason for allowing access to even a full copy of span data.

but i cann't get parent span id from SpanContext, could add a parent span id to the SpanContext?