Have a debug flag to show logging

[gebn]

Useful for seeing the error messages produced by some BMCs, e.g. collect() []error. Adding and removing logging is doing it wrong.

[gebn]

It makes a lot of sense to use prometheus/common/log, which can also add the consistent flags, e.g. for node_exporter

[gebn]

This should just be -v, which switches log's output from /dev/null to stderr. This also works across library boundaries. Reserve stdout for non-bmc specific output, so you aren't drowned in log messages on busy exporters.

[gebn]

The more modern way to do this is to allow changing the log level at runtime via an endpoint. The flag can specify an initial value.