ottyl/tickets/043-histogram-by-labels-support.md

043 — Histogram queries ignore by() grouping, inconsistent with sum/avg/rate

Summary

PromQL evaluation (evaluate_promql, src/engine.rs) has inconsistent grouping semantics across aggregation ops: sum/avg/rate respect the query's by_labels and group series accordingly, but Histogram collapses all label combinations into a single histogram regardless of by(). The same query language should not silently vary in what by() means depending on the aggregation used.

Scope

• Decide the intended behavior: either (a) support by() grouping for histograms, producing one histogram per label group, or (b) explicitly reject/ignore by() on histogram queries with a clear validation error rather than silently accepting and discarding it.
• Implement the chosen behavior in evaluate_promql's histogram branch.
• Update any dashboard panel UI (Dashboards.tsx) that lets a user combine histogram with by() to reflect the supported behavior (either offering grouped histograms or disabling by() for histogram panels).

Acceptance criteria

• histogram(metric) by (label) either produces one histogram per label group (verified by a seeded fixture) or is rejected/flagged clearly at query time — it no longer silently accepts and ignores by().
• sum/avg/rate behavior with by() is unchanged.

Depends on

None.