Code review comment for ~xavpaice/charm-prometheus2:add_default_rules

I've added a comment inline re: wait_time hard-coding.

OTOH, when the prometheus2 application is upgraded, the generic rule won't be created because of line 458:
"""
if any(config.changed(opt) for opt in rules_change_opts) \
       or templates_changed([PATHS['refresh_rules_script_tmpl']]):
        configure_rules()
"""

I think the condition should also take into consideration PATHS["generic_rules_tmpl"].

Other than the above, it looks good.