How to add multiple values in vrl

[jiaozi07]

Original log ： "upstream_response_time":"0.103,0.203"

Completed using lua

  my_lua_transform:
    inputs:
    - my_transform
    type: lua
    version: "2"
    hooks:
      process: |-
        function (event, emit)
          local upstream_response_time = event.log.message.upstream_response_time
          if type(upstream_response_time) == "string" then
            if string.find(upstream_response_time, ",") then
              local result = 0
              for i in string.gmatch(upstream_response_time, '([^,]+)') do
                result = result + tonumber(i)
              end
              upstream_response_time = result
            else
              upstream_response_time = tonumber(upstream_response_time)
            end
          end
          event.log.message.upstream_response_time = upstream_response_time
          emit(event)
        end

How to implement it using vrl ？
eventually need to be achieved
"upstream_response_time":"0.103,0.203" => "upstream_response_time":0.306