Update resource merge behaviour (#537)
Update resource merge behaviour (#537)

The resource merge behaviour changed 3 months ago to be consistent with
Span.SetAttribute. It now specifies:

> The resulting resource MUST have all attributes that are on any of the
> two input resources. If a key exists on both the old and updating
> resource, the value of the updating resource MUST be picked (even if
> the updated value is empty).

This updates sdk::Resource::merge to behave as described in the spec.
open-telemetry/opentelemetry-rust,open-telemetry/opentelemetry-rust