read-only, inherited
Extra context that can be used by for middleware and handlers.
For requests, this is used to pass data to inner middleware and handlers; for responses, it's used to pass data to outer middleware and handlers.
Context properties that are used by a particular package should begin with
that package's name followed by a period. For example, if logRequests
wanted to take a prefix, its property name would be "shelf.prefix",
since it's in the shelf package.
The value is immutable.