kvserver: add ability to filter specific request types from replica load accounting

**Is your feature request related to a problem? Please describe.**
We have seen that export requests in a lightly loaded cluster (with large amounts of data) can cause significant rebalancing activity which is ultimately wasted.

**Describe the solution you'd like**

Add the ability to exclude specific types of request from load accounting,
**Describe alternatives you've considered**

We could instead exclude these when deciding upon rebalancing decisions but still account for them otherwise, i.e. maintain separate counters for these request types.

**Additional context**


Jira issue: CRDB-38385