Add tickets to improve metric abstractions

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

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+# 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.