Metadata has been around for a few decades as a technical term, but it came into wider use over the past ten years or so in connection with the scope of government access to records of phone calls and online activities. Judging by the News on the Web corpus the usage frequency of the word is uneven around the world. For example the frequency of use in Australian news stories is 2.7 times that in Britain - presumably due to media coverage of Australian government schemes to snoop on us.
The word may be verging on common status, but I'll add it as a rare word for now.