Add a config file option for controlling the enabled feature sources,
aimed at replacing the --sources command line flag which is now marked
as deprecated. The command line flag takes precedence over the config
file option.
Add a config file option for label whitelisting. Deprecate the
--label-whitelist command line flag. Note that the command line flag has
higher priority than the config file option.
Add a new config file option for (dynamically) controlling the sleep
interval. At the same time, deprecate the --sleep-interval command line
flag. The command line flag takes precedence over the config file option.
A new special value 'all' is a shortcut for enabling all feature
sources. It should be the only name specified -- if any other names are
specified 'all' does not take effect, but, we only enable the listed
feature sources. E.g.
--sources=all enables all sources, but
--sources=all,cpu only enables the cpu source
Also, print a warning if unknown sources are specified.
