Logging mechanism for frontends

Currently any logging done by a frontend to its stdio goes to the daemon logs (for builtin and, I think, server side gateway based frontends) or to the client stdio (client side gateway frontends) where it corrupts the tty based logging.

Shall we consider adding a Log method of some sort to the gRPC provided to the frontend? So in frontend/gateway/pb/gateway.proto add:

 service LLBBridge {
         // apicaps:CapResolveImage
         rpc ResolveImageConfig(ResolveImageConfigRequest) returns (ResolveImageConfigResponse);
         // apicaps:CapSolveBase
         rpc Solve(SolveRequest) returns (SolveResponse);
         // apicaps:CapReadFile
         rpc ReadFile(ReadFileRequest) returns (ReadFileResponse);
         // apicaps:CapReadDir
         rpc ReadDir(ReadDirRequest) returns (ReadDirResponse);
         rpc Ping(PingRequest) returns (PongResponse);
         rpc Return(ReturnRequest) returns (ReturnResponse);
+        rpc Log(LogRequest) returns (google.protobuf.Empty);
 }

+message LogRequest {
+       string message = 1;
+       «enum|int|tbd» level = 2;
+       map<string, string> metadata = 3;
+} 

and then figure out a way to integrate that stream with controlapi.StatusResponse (and to display it)?

Start with frontend/gateway/pb/gateway.proto and inspect controlapi.StatusResponse to understand the existing frontend gateway and status paths. Determine how a frontend log request could be integrated and displayed without corrupting daemon or client tty logs. Done means frontend logging has a defined transport and appears through the intended status output.