Strange behaviour of --insert-count option in YCSB workload

**Describe the problem**

According the help it is a _Number of rows to sequentially insert before beginning workload_. I can assume that it either affects workloads which do insert operations or it forces some insertions before any workload.

1. In case of workload A (with updates) and workload C (read only) I don't see any insertions in metrics.
2. This flag affects results in mysterious way (concurrency 512):
* uniform workload C: 138K op/s when --insert-count and 207K op/s when 10000000.
* uniform workload A: 1K and 260K for --insert-count 1 vs 5000000
* zipfian workload A: 1K and 24K for --insert-count 1 vs 5000000

Example command:
```
cockroach workload run ycsb --workload a --request-distribution uniform --max-ops 5000000 --insert-count 1 --display-every 10s postgresql://root@localhost:26257?sslmode=disable --concurrency 512
```

In cluster I have 8 servers, each one is 128 cores, 4 NVMe disks. There are 4 cockroach instances with 32 cores (set by taskset) and 1 disk per server. Replication factor is default.

Jira issue: CRDB-25230