Side-Channel/Control Plane Information

Hey Redis Team!

I've looked through the RESP3 spec, it looks great!

One thing that sprung to mind is passing metadata in the request/response cycle. In MySQL, for example, we tend to [pass a comment in each query](https://github.com/basecamp/marginalia) with where the query came from (path, request id, ..). This is very helpful in the slow query log, and for instrumenting with e.g. BPF:

```
Account Load (0.3ms) SELECT `accounts`.* FROM `accounts`
WHERE `accounts`.`queenbee_id` = 1234567890
LIMIT 1
/*application:xx,controller:project_imports,action:show,request_id:x, ..*/
```

In the response cycle, it's possible to attach metadata as well (continuing to use MySQL as an example). We anticipate we'll soon through a plugin we will pass back information such as the CPU, Network and IO cost of a query, as well as the utilization of these resources. This will help throttle further up the stack, e.g. shed load the load-balancers or prioritize queries. We think this could be useful for Redis too, longer-term.

As far as I can understand this is _not_ something RESP3 in its current form will support. The closest I can see is Push, which is out-of-band data, that will not be tied to a query.

Curious what you think, or whether I missed something! 👀