sentry/tracing: include request-span fields in sentry transaction

[syphar]

We report server errors from two places:

• TraceLayer::on_failure reports every 5xx status
• in all the places where we return a 5xx error, we explicitly call error!() with details

But: Looking at this sentry error, coming from TraceLayer, I didn't see corresponding detailed error.

The only place where we might return 500 while not logging it is search_handler, where we return the crates.io search api status code directly to the client.

This change here will just add the span fields from TraceLayer (URL among others) to the sentry transaction, so we can see where that error actually comes from.

[syphar]

r? @GuillaumeGomez