[EXPORTER] support unix sockets in grpc client

exporters/otlp/src/otlp_grpc_client.cc

@@ -333,7 +333,16 @@ std::shared_ptr<grpc::Channel> OtlpGrpcClient::MakeChannel(const OtlpGrpcClientO
   }
 
   std::shared_ptr<grpc::Channel> channel;
-  std::string grpc_target = url.host_ + ":" + std::to_string(static_cast<int>(url.port_));
+  std::string grpc_target;
+
+  if (url.scheme_ == "unix")
+  {
+    grpc_target = "unix:" + url.path_;
+  }
+  else
+  {
+    grpc_target = url.host_ + ":" + std::to_string(static_cast<int>(url.port_));
+  }
   grpc::ChannelArguments grpc_arguments;
   grpc_arguments.SetUserAgentPrefix(options.user_agent);