"that we have before us" means nothing more than "have" or "see"
It is this necessity that we see in the elevation.
It is this necessity that you see here in the elevation.
It is this necessity that we are discussing the elevation.
It is this necessity in the elevation, which we now come to discuss.
That's all it means. It's Just That Simple. There is utterly no metaphor. Really there is no meaning, it's just a linking term.
Note that OTHER PARTS of the sentence may or may not be a metaphor (or whatever), but "that we have before us" is incredibly straightforward, it's just another way of saying "that we have here" or "that you see".
Here are some examples:
What we have before us, is a very confused piece of writing.
The site you see before us is StackOverflow.
If you see a blue background, what you have before you is a Apple
What we have before us here is a new invention.
.. and so on.
Really it's that simple.
It's an extremely common phrase and it means nothing more than that.
(Very bad, and also stupid, writers often use it to "sound important", but that's neither here nor there - and you can say that about 50% of phrases in English.)