PromQL: native-histogram accessor functions (histogram_*) unsupported #43

Summary

PromQL native-histogram accessor functions (histogram_*) parse but are rejected at lowering (UnsupportedFunction). This is the single largest un-tracked lowering gap.

Corpus impact

282 rejections (14.6% of all corpus rejections) across the PromQL corpora — the biggest un-issued category. Breakdown: histogram_fraction 181, histogram_count 35, histogram_quantile* 19, histogram_sum 18, histogram_avg 11, histogram_stdvar 9, histogram_stddev 9.

(* histogram_quantile over a classic le-bucketed vector already lowers via walk_histogram_quantile; the native-histogram histogram_quantile(φ, <native hist>) form and the other accessors do not.)

Examples

histogram_fraction(0, 0.2, rate(http_request_duration_seconds[5m]))
histogram_count(rate(http_request_duration_seconds[5m]))
histogram_sum(...)  histogram_avg(...)  histogram_stddev(...)  histogram_stdvar(...)

Notes

• Error variant: LoweringError::UnsupportedFunction.
• These operate on native histogram samples — needs a decision on the intent-algebra representation of native histograms before lowering (distinct from classic le buckets).
• Surfaced by the corpus-rejection scan over crates/lower/tests/data/*.txt.