Web console: don't send lookups to sampler

[vogievetsky]

Instead of sending transform expressions containing lookups to the sampler, which is guaranteed to give an error of lookup not found due to lookups not being loaded on the overlord, change the transform to a placeholder so it does not block the flow:

[maytasm]

@vogievetsky @abhishekrb19

Out of curiosity, why don’t we apply this logic (changeLookupInExpressionsSampling) in the Sampler API backend (SamplerResource) instead of in the frontend?

If the Sampler API is guaranteed to return an error whenever there’s a lookup in the spec, then applying the placeholder on the backend would help in all cases where the Sampler API is called, not just from the Druid Console UI. I also can't see any scenario where getting a 400 Bad Request error with confusing error message ("{"error":"Failed to sample data: Could not transform value for xxx reason: yyy encountered unknown exception."}") would be preferred to the placeholder