fix(telemetry): use resolved command/method name as span suffix

Per PR #6438 review thread r3250432621: known-command errors
(rpcTOO_BUSY, rpcNO_PERMISSION, etc.) were collapsing into a
single rpc.command.unknown span name, hiding per-command error
rates in dashboards. Same anti-pattern existed for gRPC, where
every method was bucketed under grpc.request with the method
relegated to an attribute.

- RPCHandler.cpp: doCommand error path uses cmdName as the span
  suffix; the rpc_span::val::unknownCommand fallback only applies
  when the request truly omits both command and method fields.
- GRPCServer.cpp: gRPC span name is now grpc.<MethodName>
  (e.g. grpc.GetLedger). Method also retained as an attribute.
- GrpcSpanNames.h: drop the unused op::request constant; update
  the span-hierarchy comment.
- RpcSpanNames.h: update the gRPC span diagram to match.

Dashboards on downstream phases will benefit from per-command
breakdowns without needing TraceQL attribute filters.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/xrpld/app/main/GRPCServer.cpp

@@ -169,8 +169,7 @@ void
 GRPCServerImpl::CallData<Request, Response>::process(std::shared_ptr<JobQueue::Coro> coro)
 {
     using namespace telemetry;
-    auto span =
-        SpanGuard::span(TraceCategory::Rpc, grpc_span::prefix::grpc, grpc_span::op::request);
+    auto span = SpanGuard::span(TraceCategory::Rpc, grpc_span::prefix::grpc, name_);
     span.setAttribute(grpc_span::attr::method, name_);
 
     try