cdc: have cdc/multi-table-pts-benchmark roachtests report metrics to roachperf

Currently we're asserting that the metric values of changefeed_stage_pts_manage_latency and changefeed_stage_pts_manage_error_latency are below a fixed threshold, which is noisy, especially while we have performance issues (tracked [here](https://github.com/cockroachdb/cockroach/issues/147788), #147788) for per-table pts.

We will remove this fixed threshold to prevent noise, but we want to add it to roachperf to be able to monitor the performance.

Epic: https://cockroachlabs.atlassian.net/browse/CRDB-1421
Epic: CRDB-1421

Jira issue: CRDB-54939