Implements StatsStore interface for ClickHouse using AggregatingMergeTree
tables with -Merge combinators (countMerge, countIfMerge, sumMerge,
quantileMerge). Uses literal SQL for aggregate table queries to avoid
ClickHouse JDBC driver PreparedStatement issues with AggregateFunction
columns. Raw table queries (SLA, topErrors, activeErrorTypes) use normal
prepared statements.
Includes 13 integration tests covering stats, timeseries, grouped
timeseries, SLA compliance, SLA counts by app/route, top errors, active
error types, punchcard, and processor stats. Also fixes AggregateFunction
type signatures in V4 DDL (count() takes no args, countIf takes UInt8).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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