feat(RpcServer, RpcClient): emit OTEL `rpc.*` semantic attributes

What is the problem this feature would solve?

RpcServer and RpcClient create a span per operation (RpcServer.GetUser / RpcClient.GetUser), but the operation identity lives only in the span name string. Effect does not use OTEL rpc semantic attributes (rpc.system, rpc.service, rpc.method).

What is the feature you are proposing to solve the problem?

Emit the standard OTEL rpc semantic conventions on the spans, on both sides:

• rpc.system
• rpc.service
• rpc.method

What alternatives have you considered?

• Collector-side processors to synthesize the attributes.