Which air stream pressure / speed is measured by the pitot?
The first thing to have in mind is that a pitot tube is NOT measuring speed: the pitot tube measures pressure and later on the airplane (flight management system in a modern airplane) calculates the speed based on pressure.

More specifically the pitot tube calculates the stagnation pressure which is the sum of static pressure (what you show in your picture) and the dynamic pressure (pressure obtained by stopping the air).

The total pressure remains constant in the external free flow in normal airplane operations BUT will not remain constant in the boundary layer, which is a small region close to the wall. The boundary layer grows with the distance downwash along the airplane.

What I explained here justifies the following design solutions:


Pitot tubes are usually installed with an small distance to the walls "like floating" in order to be outside the boundary layer and free of loses on the stagnation pressure.
Pitot tubes are never installed at the tail of the airplane, in order to avoid high thickness of boundary layer (could reach 1 meter).