Tezt/Cloud/DAL: echo rollup metrics

What

Adds three metrics for the echo rollup: the number of unattested slots the rollup from the beginning, the size of the total data (all slots) at the last level, and the cumulative size of data fetch from the beginning.

Why

The echo rollup serves as a witness that a rollup can handle the bandwidth, and isn't a bottleneck by itself according to the current DAL parameters.

How

At each level, look at the rollup storage and simply make the metrics.

Manually testing the MR

Let's redo the scenario from !18492 (merged). This time, we can have a look at prometheus and look the three new metrics added in Prometheus: