Pass SpanRecords by reference in Reporter trait

[andylokandy]

Hello,

The Reporter::report gets called with drained objects, but they are converted to a ref slice, which doesn't allow to own the contents of each element.
That would avoid a lot of clones in the implementations, for example you could avoid the clones here.

Thanks.

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Originally posted by @dgrr in tikv/minitrace-rust#217