-config.expand-env requires argument

[carlpett]

If not provided an argument, -config.expand-env fails parsing/validation and causes an exit. Easily reproducible with ./cortex -config.expand-env.
The working usage seems to be -config.expand-env=true.

While the documentation isn't completely clear on the topic, my reading is at least that -config.expand-env does not require any argument:

cortex/docs/configuration/config-file-reference.md

Line 31 in a87c25f

You can use environment variable references in the config file to set values that need to be configurable during deployment by using the `-config.expand-env` flag.

The various unit tests in main_test.go seem to get around this somehow?

As a side-note, passing non-boolean arguments is silently accepted as false, for example -config.expand-env=FOO would not lead to ${FOO} being expanded, but kept verbatim in the file. I tested that variety since I've used other systems in the past where a list of allowed environment variables needs to be passed, not sure if that would be a common reasoning.

[Wing924]

cortex/cmd/cortex/main.go

Line 83 in 2cbf61a

flagext.IgnoredFlag(flag.CommandLine, configExpandENV, "Expands ${var} or $var in config according to the values of the environment variables.")

It looks wrong here.
flagext.IgnoredFlag use flag.FlagSet.Var internally, which expect a value.