def []=(key, value)
      if value
        ffi_delegate.set_meta(key, "%s", :string, value)
      else
        ffi_delegate.unset_meta(key)
      end
    end
def []=(key, value)
      if value
        ffi_delegate.set_meta(key, "%s", :string, value)
      else
        ffi_delegate.unset_meta(key)
      end
    end
Set metadata.
 @param key [String] metadata key
 @param value [String] metadata value
 @return [value]