Looks to me to be "Spot" metered. On the 6D I believe spot metering is only on the center point. If you focused with the center point (like was discussed in a previous thread), and then recomposed, that would put the center point right in the clouds... and the clouds look perfectly exposed. If it was evaluative metering, then I think it would have been marginally better. But that is all speculation at this point, you can easily confirm with the EXIF data.

As others have said, you took the time to set up for the picture... so you should have put the GND filter on. I understand the comment about low quality of Cokin, but they also work and will cause less noise or distortion that you are going to get pulling the building out of the shadows in post processing. Or if you expose the building properly, you are going to lose the nice character the clouds bring to the shot. I had Cokin for many years before I got a Lee holder... and I did that to get to a 4"x6" system for wider shots. I produced many good shots with Cokin and my 7D, and I have no regrets. Cokin may not be the best, but if it is what you can afford, then it is a far better choice than trying to produce the same effect in post processing and the associated noise that comes with it. The 6D is much better than older cameras, and as cameras get better I may change my opinion on the use of GND vs PP; but that is not the case at this time (IMO).

One caveat to what I said above. If you are trying to shot 24mm or less on a FF camera, then the standard Cokin will not work - you need the wider version. Not sure which one you have.

Pat