kv.Array() does not make copy of slice

[lizthegrey]

We currently don't freeze the array that we're passed.

This has the potential to result in the naive/least verbose instrumentation doing the wrong thing, if the array that's passed in is mutated after the kv is sent to SetAttributes. This has the potential both to write incorrect data to the span, as well as to introduce a read/write slice concurrent access thread unsafety issue.