More accurate consumption metrics in presence of server restarts and replication

Currently, metrics like `REALTIME_ROWS_CONSUMED` are emitted by each partition consumer, which we can use to track real-time consumption at a topic-level by aggregating the metric per time window. However, the metric is emitted by each replica making it complicated to aggregate---ideally we would just divide the metric by the replication factor, but when replicas restart (and start re-consuming a segment) or fall-behind in consumption, this calculation is no longer accurate.

I'm wondering if there's any way to emit/calculate such a metric, accurately, at the topic-level without too much overhead.

Start by tracing how REALTIME_ROWS_CONSUMED is emitted by partition consumers and replicas, then examine how time-window aggregation could account for restarts, re-consumption, and lag. Done means a topic-level consumption metric remains accurate across those conditions without excessive overhead.

Written by the indexing model from the issue text.