Wondering if can get away without grounding it because don't know how to do that. I'm running around 10 metres of cat6a cable under the floor in a metal conduit. Both ends terminate in 6a keystones

You will be fine

No, but if it is SFTP or even FFTP the outer shield can act as an antenna for noise, and so can metal conduit, and if you don't ground it, some of that noise will end up in your data. 99.999% chance it won't have any effect especially at only 10M, but technically it is supposed to be grounded to avoid that.

FTP (foil around each pair only) and obviously UTP/unshielded don't require it. Some still prefer to ground FTP but it is a pain and not really necessary.

When you do ground it, you ground one end only, typically the one at the switch/router/patch panel end.

I didn't know you only ground one side!

is grounding the backbox enough? It's metal backbox that the keystone goes in

If you ground both ends you can end up with a ground loop, which causes more interference. Just ask anyone who owns a Chrysler product

Yup, otherwise you create a ground loop. That's a bit more likely to actually have an impact on errors and throughput, can actually be worse than not grounding at all.

No because the keystone isn't grounded to the box.

If you want to ground it, first make sure one of the metal boxes or the conduit is grounded to your power system's ground, then just attach the braid from the cable to the box (you can strip the outer jacket back as far as you need, just keep the pairs twisted). If it is foil shield, same idea, it is just more delicate so you have to be a bit more careful.

You don't need to ground the individual foils around the pairs, those will ground via the main outer shield (assuming it isn't FTP without an outer shield).

you can ground it but you don't need to unless it is running near something that would cause interference like a high voltage power line

Technically correct solid answer here. Kudos.

For a 10 m home run, the practical answer depends on what cable you actually bought.

If it is UTP Cat6A, there is nothing to ground. Just terminate it normally.

If it is shielded cable, grounding is about the shield, not the copper data pairs. The clean way is: shielded cable -> shielded keystone/patch panel -> one proper building ground path. Do not just randomly tie both ends to whatever metal is nearby, because a bad ground path can create more trouble than it solves.

The metal conduit is a separate question too. If the conduit is part of an electrical grounding system, fine. If it is just a loose metal sleeve under the floor, do not assume it is a useful ground.

For your exact case, I would first check the jacket marking. If it says U/UTP, install it and move on. If it says F/UTP, S/FTP, SF/UTP, etc., either use matching shielded hardware and ground it correctly at one end, or swap to unshielded cable if this is just a normal indoor home network run. At 10 m, unshielded Cat6A is usually the simpler choice.

This guy grounds. +1

You're mostly overthinking it.