[Task] Finish the Netty instrumentation

[EddeCCC]

The new version of the Spring Petclinic uses Netty for sending HTTP requests with the api-gateway to other services.
There is already some instrumentation in the demo to collect traces and service-graph data.

However, some parts of a complete instrumentation are still missing. In particular:

• Collecting method duration metrics for the requests
• The service graph does not show request durations or error rates
• Only traces for GET-requests of the api-gateway are down propagted. Other HTTP methods create seperated spans

We can reconsider if we use the appropiate scopes and entry- or exit-markers.