fix otlp database span resource

[SylvainJuge]

With OTLP intake, the database spans always have their span.destination.service.resource set to {span.subtype}.

However when using our APM agents it's captured and sent as {span.subtype}/{db.name}, for example mysql/my-database.

This PR simply fixes the mapping for database spans.

Reported in our forum here.

The part where I'd like to have input from APM-server team is about the initial jaeger input which the tests refer to.

• if a resource was already set (like in the jaeger test data), then this would potentially reduce the cardinality or at least change the current resource field value (so minor change visible in the map/dependencies).
• if a resource was not already set, then this will just increase cardinality by a bit by providing some per-database cardinality.