drpc: flatten interceptor call chains when possible

In gRPC, interceptors are chained together — a behavior that's crucial for many use cases. For example, latency metrics need to capture the full duration of the call chain, and the same applies to distributed tracing. However, these deep interceptor chains consume significant stack frames (in the profile) and create noise during debugging or incident analysis. As part of the DRPC integration effort, we should explore opportunities to _flatten the interceptor call chain when possible_.


Jira issue: CRDB-51650