It's a bit more than a metaphor :) during the California gold rush there was this guy named Sam Brannan who sold shovels and other tools to miners, and made a fortune from it (he is often referred to as California's first millionaire). He also had a newspaper at the time, the California Star, which as you can imagine was used to promote the gold rush:
> The excitement and enthusiasm of Gold Washing still continues—increases. (1848)
Note that it's a pretty common cliche, usually phrased something like "in a gold rush, the only people guaranteed to make money are the guys selling the shovels".
