TraceInterceptors functionality to the SDK

Is your feature request related to a problem? Please describe.
Hello :)

Currently, we are in the process of migrating from DD (DataDog) instrumentation to Opentelemetry. During this migration, we have encountered a specific case where we want to implement TraceInterceptors. In other words, we want to add custom functionality at the end of a specific trace.

Here's a simple example: we would like to have a TraceInterceptor that collects the number of spans for that particular trace and then adds this number as an attribute to the root span. This is just an example

Datadog Instrumentation, provides that feature with the TraceInterceptor interface (Ref)

Is there something similar to the sdk that we can use to achieve the same functionality ?

Describe the solution you'd like
The solution that we would like is to have an Interface that we could utilize to: add our custom implementation at the end of a trace, and have all the information (Spans, SpanData etc) for the trace.

Describe alternatives you've considered
The alternative that we have considered is using the SpanProcessor. However its not possible to know when the Trace has ended as it applies only to spans.

Thanks in advance for your help, comments

Start with the SDK tracing APIs and the SpanProcessor alternative linked in the issue, then compare them with DataDog's TraceInterceptor interface. Determine how trace completion and access to spans or SpanData should be defined. Done means a reviewed SDK design or implementation that supports custom functionality after a trace ends.