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

[kvoli]

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