Default value for `config.file` not honored with explicit `web.listen-address`

It seems that adding the --web.listen-address without also setting --config.file fails to correctly read the default configuration file.

In other words both or neither need to be set for things to work. If only --web.listen-address is set:

curl "http://127.0.0.1:9116/snmp?auth=auth-5&module=foo&target=127.0.0.1"
Unknown auth 'auth-5'

We've been running it in docker like this:

docker run -it --rm \
-v /etc/snmp-exporter:/etc/snmp_exporter:ro \
--network=host \
prom/snmp-exporter:v0.25.0

and we hit this problem when adding --web.listen-address like this:

docker run -it --rm \
-v /etc/snmp-exporter:/etc/snmp_exporter:ro \
--network=host \
prom/snmp-exporter:v0.25.0 \
--web.listen-address=127.0.0.1:9116

workaround like this:

docker run -it --rm \
-v /etc/snmp-exporter:/etc/snmp_exporter:ro \
--network=host prom/snmp-exporter:v0.25.0 \
--web.listen-address=127.0.0.1:9116 \
--config.file /etc/snmp_exporter/snmp.yml

This isn't a big problem in my case as I can simply set --config.file and be done with it. It took me a while of head scratching to figure this out though.

Start by reproducing the two Docker invocations shown and trace how --web.listen-address and the default --config.file are handled. Done means setting an explicit listen address still loads the default configuration file without requiring --config.file; the issue names no source files or tests.