Being able to associate resources with spans and not only a with a tracer

Is your feature request related to a problem? Please describe.
I would like to create spans and associate custom resources to that span. Currently I cannot call setResource() on the span builder to achieve this. Instead internally the resource associated with the tracer is used and can only be overridden on tracer level. In my case I do not have microservices but kind of virtual processes (not physical ones represented as threads) and do not want to create separate tracers for each virtual process (could be thounsands). Instead I would like to share the tracer and just adapt the resource on span creation.

Describe the solution you'd like.
I would like to have a setResource() method on span builder to set the resource used.

Describe alternatives you've considered.
As a workaround I am using setAccessible() on the resource field of a span.

Start with the Java SDK's SpanBuilder and the existing tracer-level resource handling. Trace how a span receives its resource and determine the expected precedence when a builder resource is set. Done means SpanBuilder exposes setResource() and spans created with it use the supplied resource without requiring separate tracers.