Auto-wrap ratio metric denominators in NULLIF

[shangyian]

Summary

Division-by-zero in metric SQL surfaces inconsistently across engines (Spark returns NaN, Trino raises an error, Postgres returns Infinity). The fix is to auto-wrap the right-hand side of every divide in the metric's combiner expression with NULLIF(denominator, 0), in two places:

• Auto-built combiners from decomposing metrics like AVG
• User-authored combiners when it's a user-defined ratio metric

A rate computed over zero observations is mathematically undefined; NULL represents that correctly and propagates safely through downstream aggregations. The auto-wrap normalizes divide-by-zero behavior across Spark/Trino/Postgres/Druid, and closes the class of "data unavailable" / NaN-percentile bugs without requiring every consumer to re-implement the same patch.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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