A center post would interfere with the use of the space covered by the pergola and look terrible, in my opinion. What you have will sag a little, but not enough to make it look bad, or justify adding something to it that will detract from the design. It probably won't sag much more that it already has naturally, but, if it bugs you in the future, you could disconnect the knee braces, jack up the center of the spans somewhat higher than you want them to be, and reattach the knee braces to support this higher center position. If you do it right, the center will settle down in about the right place once you remove the jack.

My advice is to leave it be for now, and consider planting grape vines on the short sides that will give you shade in the summer, and hide any sagging as the vines climb over the structure. And you'll get fruit!