Server::trace_fn should receive full request, not just headers

[polivbr]

Feature Request

Crates

tonic

Motivation

While the Server::trace_fn method allows a span to be created for each request being serviced, there is not enough information to create the span with meaningful data. For example, I would like to name the span after the request path to identify which service method is being invoked. However, trace_fn only has access to the headers, which don't contain this data. If trace_fn was instead passed the entire request, the request path (and a lot more) would be available for span construction.

Proposal

trace_fn should be invoked with the entire request, not just the headers. Unfortunately, this will require an API change.

Alternatives

I haven't attempted it, but it should be possible to use a full-blown interceptor as an alternative, albeit with a lot more work.

[davidpdrsn]

I wonder if there is a reason the API wasn't designed like that to begin with 🤔 Could just be an oversight in which case I think it would be a good change.

I haven't attempted it, but it should be possible to use a full-blown interceptor as an alternative, albeit with a lot more work.

I don't think its possible to do with interceptors as they can only return new requests. There isn't a way for an interceptor to return a span to tonic for it to manage.

I think it would be possible using tower to make a middleware that creates a span an manages it itself but it would require quite a bit of boilerplate.