Tracing context optimization and a new label filtering API

[MOZGIII]

This PR adds another way of filtering labels, and generally rewrites the internal label storage to be more optimal.

To do:

• [ ] ensure that the SmallVec actually improves anything - post bench results here

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Related: https://github.com/timberio/vector/pull/6222

[tobz]

@MOZGIII Whenever this is ready and you have a chance, it'd be good to have a 2-3 sentence summary of the changes, and some rough before/after numbers.

[tobz]

Going to close this as it's long out-of-date.