MatchedPath is empty on fallback

[edelanghe-ledger]

When using a axum fallback, the service OtelAxumService will record an empty http.path and only the method in otel.name.
The reason is that MatchedPath is empty since it doesn't have any pattern to match.

It's basically the same issue that was encountered with nested path in this previous issue: #54

Could we bring back the related fix : fe2cee7 ?

It could be configurable under a feature flag if needed.
The best would be for users to be able to record an handler that will extract the http route from Request as they want.

[davidB]

The current behavior, is to be compliant with the spec: https://opentelemetry.io/docs/specs/semconv/registry/attributes/http/ (search for http.route)

The matched route template for the request. This MUST be low-cardinality and include all static path segments, with dynamic path segments represented with placeholders.

otel backend/storage can index on this route, so if someone call with random route, it will be handled by fallback, and have a bad side effect on on your otel backend/storage. It's one of the why(s) only "matched" route are used.

but maybe, we could add url.full as part of the span records: https://opentelemetry.io/docs/specs/semconv/registry/attributes/url/

[edelanghe-ledger]

I understand the issue.

Adding url.full will not solve my issue as my otel backend (datadog) won't use it to index.

So what about adding a way to configure the http_route handler when we build the OtelAxumLayer ?

We can add a RouteParser trait and keep the current impl. Then users can replace the RouteParser by one of their own

[davidB]

Do you want to do the PR?

The documentation should warm the user about overriding the behavior (issue on wrong implementation, side effects against malicious request/DDOS,...)

[edelanghe-ledger]

I will give it a try soon, I hope.