Observation instrumentation incorrectly manages parent observations

[bclozel]

The GraphQlObservationInstrumentation is incorrectly setting up the parent observation for both request execution and data fetching observations.

In the case of the request execution, we are not looking into the GraphQL Context for an existing observation - if it exists, it needs to be set as the parent. As for the data fetching observation, we're incorrectly assuming that the parent of all data fetching operations is the request execution one - data fetching operations can be nested and we should only rely on the current observation in the GraphQL context.