fix(akka): improve handling of http.route

[droidnxs]

Fixes #15503

1. On JDK 11, route matcher does not capture the http.route attribute at all in some cases. As a result, the span name only encapsulates the HTTP method (e.g. GET).
2. The span name and http.route attribute are neglecting slashes when computing the route. This is reproducible on both JDK11 and JDK21.

Following the recommendation on slack, I chose to follow a similar approach to the pekko instrumentation's route capture logic.

[linux-foundation-easycla]

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• ✅ login: laurit / name: Lauri Tulmin (9cbf98b, e7cbd06, f3457bb)

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