otelmongo: Various attributes improvements

[matej-g]

Noticed that that there are couple of attributes-related areas in otelmongo that could be improved:

• Reconsider usage of service.name attribute - according to the specification this is a resource attribute, not a trace attribute (see When should an instrumentation library set a service name attribute? #454 (comment))
• There is a significant overlap between attribute keys in db.go / tags.go and semconv package - preferably, we should use the "standardized" ones from go.opentelemetry.io/otel/semconv package
• Go over https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/database.md and look if any of the required / recommended attributes are missing

[XSAM]

We should also improve the span name of otelmongo. It only uses mongodb.query, which is not fit with the specification.