fix memory leak cause by the spanStore.(census-instrumentation/opence…

[NanoRed]

…nsus-go#1245)

• when enable the spanStore and call startSpan and End in a loop, it cause memory leak. Found that it delete the wrong ptr in active set of spanStore in span's End() Method

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[NanoRed]

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[dashpole]

Ah, I think I broke this when I added the SpanInterface. I think we can fix this by changing the spanStore methods to use the SpanInterface instead of the *Span

[NanoRed]

sounds reasonable