Cull and document WebPageTest's logging.[type] calls, to better-understand/use them

[stephendonner]

Cull the various logging.[type] calls[0], in the hopes to catalogue and better-understand/document them:

logging.debug
logging.critical
logging.exception
logging.info
logging.warning

More than likely, we'll increasingly want to augment, change, etc., some of these for Firefox + Marionette, while we work through test-run stability and high(er) metric & metadata fidelity.

[0] https://github.com/WPO-Foundation/wptagent/blob/3f2128a9815838f462187b870be3c666ebd13d95/internal/support/visualmetrics.py#L1750-L1758