admission: AC roachtests should all be asserting on some metric

Currently, tests (such as `admission-control/index-backfill`) don't assert anything and are just used as internal benchmarks to be run manually to observe behavior.

It would be useful to have these tests catch any regressions automatically.

- [ ] admission_control_database_drop.go
- [x] admission_control_disk_bandwidth_overload.go
- [x] admission_control_disk_iops_overload.go
- [ ] admission_control_elastic_backup.go
- [ ] admission_control_elastic_cdc.go
- [x] admission_control_elastic_io.go
- [x] admission_control_elastic_mixed_version.go
- [ ] admission_control_follower_overload.go
- [ ] admission_control_index_backfill.go
- [ ] admission_control_index_overload.go
- [x] admission_control_intent_resolution.go
- [ ] admission_control_multi_store_index_backfill.go
- [ ] admission_control_multi_store_overload.go
- [x] admission_control_multitenant_fairness.go
- [ ] admission_control_row_level_ttl.go
- [ ] admission_control_snapshot_overload.go
- [ ] admission_control_snapshot_overload_io.go
- [ ] admission_control_tpcc_overload.go

Jira issue: CRDB-32481

Start with the unchecked admission_control_*.go roachtests listed in the issue, using checked files such as admission_control_disk_bandwidth_overload.go as comparison points. Determine which metric each test should assert and update the remaining tests so they automatically catch regressions. Done means the unchecked tests no longer rely only on manual observation and all listed items have assertions.

Written by the indexing model from the issue text.