Fetch time-series session metrics for the observe graph.

On trace, span, session, graph, and eval-task bounded reads, created_at/start_time datetime filters support equals, greater_than, greater_than_or_equal, less_than, less_than_or_equal, between, not_equals, not_between, is_null, and is_not_null. Missing bounds retain the finite default window: 30 days ago for the lower bound and request-time now for the upper bound. Between and not_between use half-open [start, end) ranges; not_equals excludes one DateTime64(6) microsecond. Because the physical created_at/start_time field is non-null, is_null returns an exact empty result without a ClickHouse read and is_not_null preserves the base window. Valid contradictions also return an exact empty result.