Depending on what you drink (coffee, tea, etc) anything that leaves a stain on your metal can easily be cleaned once a month or more with some diluted bleach and warm water (I typically just eyeball it, but if you want to measure it out it’s maybe 20:1 water:bleach)

Let sit for 10 minutes and rinse out multiple times with dish soap and scrub as best as you can. It’ll come out brand new just from the bleach soak. The hardest part is getting the bleach scent out, which a couple rinses with dish soap takes care of easily