There are multiple ways of associating metadata in plaintext files, depending on the file format:
The Front matter (used in Nanoc, Jekyll, …) can be used for any plaintext file format. It’s commonly YAML, but can also be TOML.
AsciiDoc has its own metadata format.
HTML has meta tags.
Nanoc only supports front matter natively (with YAML), though with some preprocessor logic, other types of metadata can be extracted.