Add tickets to improve metric abstractions

tickets/042-fix-sum-avg-timestamp-merge.md

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+# 042 — Verify/fix exact-timestamp merging for non-rate sum()/avg()
+
+## Summary
+
+Non-rate `sum()`/`avg()` queries merge per-label series via
+`merge_series_exact` (`src/engine.rs`), which only combines values whose
+timestamps match *exactly*. In practice, raw occurrence timestamps across
+different label groups essentially never land on the same millisecond, so
+`sum(requests) by (path)` over raw occurrence counts likely produces sparse,
+near-empty merged series rather than a meaningful "total requests over
+time," even though that's the obvious expectation for such a query. The
+existing code comment justifying exact-match merging ("nothing to carry
+forward") only holds if the query isn't meant to sum sparse counts over
+time — which is exactly the common case for occurrence-count dashboards.
+
+## Scope
+
+- Build a seeded fixture with multiple label groups whose events arrive at
+  different (non-coincident) timestamps and confirm whether
+  `sum(...) by (...)` currently produces the expected combined total or the
+  suspected sparse/near-empty result.
+- If confirmed broken: bucket points into fixed time windows (the same way
+  `rate()` already windows) before summing/averaging across label groups,
+  instead of requiring exact timestamp coincidence.
+- Add a regression test covering multi-label-group `sum()`/`avg()` over
+  time.
+
+## Acceptance criteria
+
+- `sum(metric) by (label)` over multiple label groups with staggered event
+  timestamps produces a sensible combined time series (not sparse/empty),
+  verified by a new test fixture.
+- `avg()` behaves analogously.
+- Existing single-label-group queries are unaffected.
+
+## Depends on
+
+None.