[enhancement] Add debug mode in monitoring config

[devkapilbansal]

Yes, I remove -t flag when logging in verbose mode. In verbose_mode, it is automatically tagging the logged message by the procd service name and if I pass a tag, then the message is logged twice

What do you think about adding a new "debug mode" to turn this on?

https://github.com/openwisp/openwrt-openwisp-monitoring/blob/bac6d545b0e3f21f2ca6d7e409220024963f111a/openwrt-openwisp-monitoring/files/monitoring.init#L89

In verbose mode we could avoid using it. In debug mode we would know (and document) that any output is sent to the logging facility, causing some output to be repeated, Or alternatively, we could even modify our log function to use echo instead of logger.

See https://github.com/openwisp/openwrt-openwisp-monitoring/issues/57#issuecomment-922500877