Store retries count in http::extension::Extensions

[aimericsr]

Motivations

I tried to register a new field http.request.resend_count to my Otel trace but I don't have access to this information by implementing ReqwestOtelSpanBackend.

Solution

Register the number of retries in the extension of the current request.

Alternatives

I tried to inspect the logs message of the current Span inside the on_request_end() function to get this data from the
warn event but it's tedious and I don't think it's even possible.

Additional context

Here is my current code

impl ReqwestOtelSpanBackend for OtelTrace {
    fn on_request_start(req: &Request, extension: &mut Extensions) -> Span {
        let otel_span_name = format!("{:#?} {:#?}", req.method(), req.url().path())
            .trim()
            .to_owned();
        reqwest_otel_span!(
            level = Level::INFO,
            name = otel_span_name,
            req,
            url.full = req.url().to_string(),
            http.request.resend_count = tracing::field::Empty,
        )
    }

    fn on_request_end(span: &Span, outcome: &ResultReqwest<Response>, extension: &mut Extensions) {
        // This would be someting like this
        let retries = extension.get::<RequestRetries>().unwrap().count;
        span.record("http.request.resend_count", retries);
        default_on_request_end(span, outcome);
    }
}

I could take the PR if it sounds good for you.