Reduce allocations

[boekkooi-fresh]

Hello,
First of thank you for taking the time to view this PR and your work on opencensus!

While trying to find a performance issue on a service I noticed that Span.lruAttributesToAttributeMap was allocating a bit more often then I was expecting.
To reduce the amount of allocation I now provide the length of the source slice/map as the capacity as the new map.slice.

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[boekkooi-fresh]

@googlebot I signed it!

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[rghetia]

@boekkooi-fresh thank you for finding the issue and fixing it.

[boekkooi-fresh]

No problem 😄 and thanks for the quick merge.