LGTM.
But I may have a question that if you will add more to this \xff\xff/tracing in the future?
If so, maybe adding a nested prefix here is better, like \xff\xff/tracing/<some_prefix>.
In the future, if you have a different read logic on a range \xff\xff/tracing/<future_prefix>, it is easy to add.
If you think this is enough, then it is good to use \xff\xff/tracing

LGTM.
But I may have a question that if you will add more to this \xff\xff/tracing in the future?
If so, maybe adding a nested prefix here is better, like \xff\xff/tracing/<some_prefix>.
In the future, if you have a different read logic on a range \xff\xff/tracing/<future_prefix>, it is easy to add.
If you think this is enough, then it is good to use \xff\xff/tracing

I don't think there will be more keys added to the top level tracing/ layer, as the only control clients need to have over tracing is to set a custom identifier or to disable tracing for the transaction. Maybe there will be support for an action like reading a log specific to a span, but in that case the request would look like get \xff\xff/tracing/<span-id>/logs, reading data specific to a span. I guess if that occurred, it could change the range read behavior of reading the range \xff\xff/tracing/ - \xff\xff/tracing0?

LGTM.
But I may have a question that if you will add more to this \xff\xff/tracing in the future?
If so, maybe adding a nested prefix here is better, like \xff\xff/tracing/<some_prefix>.
In the future, if you have a different read logic on a range \xff\xff/tracing/<future_prefix>, it is easy to add.
If you think this is enough, then it is good to use \xff\xff/tracing

I don't think there will be more keys added to the top level tracing/ layer, as the only control clients need to have over tracing is to set a custom identifier or to disable tracing for the transaction. Maybe there will be support for an action like reading a log specific to a span, but in that case the request would look like get \xff\xff/tracing/<span-id>/logs, reading data specific to a span. I guess if that occurred, it could change the range read behavior of reading the range \xff\xff/tracing/ - \xff\xff/tracing0?

yeah, adding \xff\xff/tracing/<span-id>/logs, sounds like a behavior change(an API change), which I think if you want to add it, you may need to wait until the next major version release(if the current major version has already been used) and update the api notes. Is it okay?

I'm speculating about possible future features -- however, in general I think most span state should be internal and the transaction shouldn't need to access it. In that case, the above example wouldn't need to be added. @sfc-gh-mpilman do you have thoughts on this?

Sorry for misleading, I just confirm the contract with Andrew for what is a behavior change in special keys.
So adding anything new inside a module(a.k.a. \xff\xff/tracing) is considering a behavior change.
So anything you plan to add to the tracing module in the future will be a behavior change and need a major release for it.
Thus, I think adding a top-tier prefix doesn't help anything here.
(I was taking it wrong as adding non-overlapping ranges are not considered API change.)