metrics is a list of named metric expressions. Each expression must have at least one
metric reference, and every metric reference must carry a rollup (e.g. avg, p95).

MetricsFilter narrows the query scope.
Time range constraints: the window [from_time, to_time) must not exceed 168h (7 days).
from_time must not be more than 30 days in the past.
When from_time is unset, defaults to (now - default lookback of 24h).
When to_time is unset, defaults to now.

Filter resolution: some fields (namespace, kind, name, container, is_custom) are
pushed down to ClickHouse directly. Other fields (scaling_policy_name, label,
workload_id) are resolved in PostgreSQL first, and the resulting workload list is
passed to ClickHouse. OR branches that mix PG-only fields with CH-pushable fields
are rejected because the two-stage pipeline would incorrectly rewrite them as AND.

MetricsAttributes defines how results are bucketed and grouped.