Use generator expression for OPENTELEMETRY_PROTOBUF_LIBRARY and OPENTELEMETRY_PROTOC_EXECUTABLE so it works with FetchContent targets too

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -4288,10 +4288,8 @@ macro(build_opentelemetry)
 
   get_target_property(OPENTELEMETRY_PROTOBUF_INCLUDE_DIR ${ARROW_PROTOBUF_LIBPROTOBUF}
                       INTERFACE_INCLUDE_DIRECTORIES)
-  get_target_property(OPENTELEMETRY_PROTOBUF_LIBRARY ${ARROW_PROTOBUF_LIBPROTOBUF}
-                      IMPORTED_LOCATION)
-  get_target_property(OPENTELEMETRY_PROTOC_EXECUTABLE ${ARROW_PROTOBUF_PROTOC}
-                      IMPORTED_LOCATION)
+  set(OPENTELEMETRY_PROTOBUF_LIBRARY "$<TARGET_FILE:${ARROW_PROTOBUF_LIBPROTOBUF}>")
+  set(OPENTELEMETRY_PROTOC_EXECUTABLE "$<TARGET_FILE:${ARROW_PROTOBUF_PROTOC}>")
   list(APPEND
        OPENTELEMETRY_CMAKE_ARGS
        -DWITH_OTLP_HTTP=ON