Contextual information from interceptors to request handlers

### Is your feature request related to a problem? Please describe.

This might already be possible but I haven't found a way to do it. Please advice.

What I would like to do is to pass additional, per-request information from the interceptors to the request handers. For example, I've an auth interceptor which validates the authentication params and I would like to attach some information (e.g. the user id) that's latter consumed by the request handler.

### Describe the solution you'd like

A way to attach contextual information to requests.

### Describe alternatives you've considered

- I can think of moving some of the logic outside of the interceptor into another module/function and call that from the request handlers. While it would work, it's not as clean as adding the data I need in the interceptors.
- I could also whatever information I want to the request metadata. I don't like this one so much because the boundaries between client metadata and my metadata are blurred. ~~I could use a symbol to avoid clashes though~~ (can't pass a symbol as metadata keys must be strings). And metadata only allows buffers or strings as values and I'd like to be possible to pass an object.

### Additional context

This is possible in many other http api frameworks.