I5 4690 should actually be still ok for 1070ti for most cases.
Only some small numbers of games have issues with 4690 e.g. BF1.
The max you can go while keeping your cpu platform is only to go to I7 4790k.
If you want better processor, you need to move to other newer platform e.g. coffeelake or ryzen.
If you choose to move into newer platform, you will need a new mainboard and RAM kit along with your processor.
i have i5 4690, stock clocks, 1070Ti 8gb, and i don’t have an issue with bottleneck even in metro exodus, so u have other problem, but i don’t know.
btw if u need to change cpu, do what im gonna to do, look for i7 4790k