tools/rados: Remove plain text snippets from rados bench JSON output

tools/rados: Remove plain text snippets from rados bench JSON output

`rados bench` emits performance stats as its output. It is very helpful
for this output to be in a machine-readable format and the CLI provides
the `--format=json` flag to achieve this.

There are some logs that do not respect the formatter flag though, as
they provide status updates as the tool is running and do not form part
of the output dataset. This prevents the contents of stdout from being
valid JSON which destroys the machine-readability of the output.

To resolve this we gate those status messages behind a check for the
formatter. If any specific formatter is provided we do not emit the
status logs. This leaves the plaintext output largely untouched while
helping the machine-readable output to be well-formed.

Fixes: https://tracker.ceph.com/issues/74370
Signed-off-by: Jacques Heunis <jheunis@bloomberg.net>