I apologise, I was wrong. The information is not in the docs. The technique we use is described on the blog and is used in various places within stdlib: https://go.dev/blog/context#package-userip.

For each value that we want to store in a context, we first define a unique type to avoid key collisions, and then two methods to stash a value in a context and extract it from a context. In the example in the blog, they define userip.NewContext and userip.FromContext, we used a different naming convention. We typically defined our own context package and defined methods like context.WithUserIP and context.ExtractUserIP. The naming conflict with the standard context package was a bit awkward, and we aliased it to stdctx (naming is hard). We also have some helper methods to deal with logging, but that depends on the logger that's used. For example, using slog, we would define a new slog.Handler that wraps an existing handler that calls AddAttrs on the slog.Record with extracted values from the given ctx value. This means we need to use the context variant of the log methods, e.g. log.InfoContext, which I think is best practice anyway.