Depends on the metal, you don't want it to look like scaffolding, you can do more with wood.
Not an arch, but something similar. I built this about thirty years ago, to complement my koi pool, Japanese lanterns and tea-house.
But of course it eventually rotted like most wooden garden structures, "from the feet up."
We've two pergolas that have been up over 25 years that are wooden. They are pretty heavy duty, this, (as can the other) can easily take my weight if I need to get on the flat roof extension of our lounge. They are fixed to a beam on the back wall above the French windows The single support posts are 16ft apart. Can't afford them to rot.
But the feet of the support posts are protected.
The bases sit on and are screwed to a small bit of terrazzo which in turn is screwed to the York stone patio. I made, "elephants feet" out of fine mix concrete, with a dyed skim of mortar. There's a bead of clear silicone around where the wood enters the foot, to stop any water penetration. The tops of the posts have plywood caps screwed to them with silicone between them and the cross beams. All painted with Dulux exterior Woodsheen.
So no rot.
If you choose a wooden arch you could try something similar.
