Prior to this commit, GraphQL Request Observations would not record
errors as Observation errors, because with GraphQL errors can partially
affect the response and there can be multiple. Instead, an invalid
request (for example) would lead to a `"graphql.outcome", "REQUEST_ERROR"`
low cardinality KeyValue. In this case, developers would not know what
type of error occured nor if there were multiple.
This commit records all errors listed in the GraphQL as
Observation.Event on the request Observation. Such events are usually
handled by the tracing handler and are recorded as span annotations for
traces. Other `ObservationHandler` annotations can leverage events in a
different fashion.
Closes gh-859
