o11y: protected timestamp record graph may be misleading

As discovered through a DRT investigation, the Protected Timestamp Records graph in the DB Console may be misleading. We see this chart:

According to @stevendanna:

> The graph records the number of "protected timestamp records" but it does it via the kv subscriber and the thing it seems to actually count is number of span configs that are covered by a protected timestamp.

>So, in this case, I think it is going to end up report 3x the number of span configs we have

The last part is due to the fact that in the given case, we have 3 system-level protection policies.

More details can be found [here](https://cockroachlabs.slack.com/archives/C05FHJJ0MD0/p1705684617791209?thread_ts=1705671015.242739&cid=C05FHJJ0MD0).

Jira issue: CRDB-35446