Add trace.WithAllowKeyDuplication

[pellared]

More under #8471 (comment)

The option was reverted in 5fa1035

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[Dipanshusinghh]

Looked at 5fa1035 — allowDupKeys was checked at four hot-path sites in span.go (SetAttributes, addOverCapAttrs, addEvent, AddLink), adding branch cost on every span operation even when never set.

Since #8471 already has a dedupAttr helper, a cleaner approach would be swapping the function pointer once at TracerProvider construction (like sampler/idGenerator) to keep the hot path branch-free. Happy to prototype — could I get assigned?

[pellared]

@Dipanshusinghh, nice idea! I assigned you.

Please also make sure to add benchmarks when working on the issue.

Later you can also tackle #8479

[MrAlias]

Since #8471 already has a dedupAttr helper, a cleaner approach would be swapping the function pointer once at TracerProvider construction (like sampler/idGenerator) to keep the hot path branch-free.

Definitely worth trying. Please make sure to show that the anonymous function lookup does not cost more than the branching would.