The default s3_object_key_format is racy with multi workers

[daipom]

Is your feature request related to a problem? Please describe.

I assume the same problem could occur in multi-workers as #326, although I have not confirmed the problem.

In #326, multi-threading with flush_thread_count is discussed,
but I think the same problem exists when running with multiple workers.

Describe the solution you'd like

In #327, the warning was added for the case of multi-threading with flush_thread_count.
So we need to add the same warning for the case of multi-workers.

Describe alternatives you've considered

None.

Additional context

No response

[daipom]

#392 is merged, so I close this.