Offer /config endpoint

[RichiH]

One consequence of #34 is that we will need a lookup table for special values. It seems least bad to make it easy for operators to reference to those values directly in the config of the exporters. To allow operators to do this, we need a /config endpoint.

This also mandates secret blanking.

It might make sense to put this functionality in https://github.com/prometheus/exporter-toolkit instead of a specific exporter. @SuperQ might be willing to do the actual work.

HELP strings would also be an option, but they are currently not persisted by storage and they might get arbitrarily long.

Still, "both" is probably most resilent.