[mongodb_exporter] log everything to stderr

mongodb_exporter store some log files in /tmp directory, this directory can be changed by -log_dir option.
also something logged to /var/log/pmm-mongodb-metrics-*.log

I think we should have the same approach for all our exporters:
don't create any log files, just print everything to stderr (like node_exporter and mysqld_exporter do).
in this case we receive all logs in /var/log/pmm-mongodb-metrics-*.log file and it will be possible to run several mongodb exporters on one host