Handle root spans that are both incoming and outgoing calls - e.g. nginx

Continuing the discussion from https://github.com/elastic/opentelemetry-lib/pull/125#discussion_r1863663594

https://github.com/elastic/opentelemetry-lib/pull/125 addressed the default case of root spans that represent a dependency.

There are some (I'd argue non OTel/SemConv compliant) cases where the check we added may not identify the span as a transaction:

```
isExitRootSpan := s.isTransaction && span.Kind() == ptrace.SpanKindClient || span.Kind() == ptrace.SpanKindProducer
```

One known case is nginx where nginx behaves as a proxy and creates a single HTTP span for those - where span kind may not be client.

Question is how we address those. So far the option on the table is to also take attributes into consideration in the code above.