Allow large step batch size with duplicate label check enabled

[yeya24]

Today, due to the current implementation of duplicate label check operator, steps batch size cannot exceed 64.

This is more a limitation of the implementation. Ideally, duplicate label check operator should work regardless of the steps batch size. We can use a different operator implementation if step size is > 64, even though the performance is worse than the current implementation.

[umegbewe]

Hi @yeya24

So i dug into this issue a bit, per my understanding the 64 step limit comes from the current bitmap approach (uint64 used to track per-step presence), so anything > 64 steps can't be represented. There's also an assumption that all samples for a timestamp arrive in a single batch, which isn't true once selectors batch series, so duplicates can span multiple batches for the same timestamp and get missed.

I'm thinking a fallback for StepBatch > 64 that switches to a slower map based check that tracks label hashes per step and works regardlesss of batch size, in addition a timestamp tracker so state resets only when the batch timestamps change which fixes the cross-batch timestamp issue

Happy to send a PR if this direction looks good.