Per-topic consumer lag for a pipeline

Current consumer lag per topic (default), or lag at a point in time when at is given. Values are totals: one sample per partition, summed per topic and across topics, so they can be reconciled with alerts. at within the last 2 days is served at minute resolution, within 14 days at hour resolution; older or future values return HTTP 400. When no sample exists near at the response carries resolution: null and a message instead of zeros. latency_ms is best-effort and only available in current mode. at without a timezone is treated as UTC. Pipelines show only the topics the destination consumes for this pipeline: transform output topics when the pipeline has transforms, otherwise its source topics. Lag is a property of the destination consumer group, so a topic shared by two pipelines into one destination reports the same lag in both.