Just adding one more thing, I had very good lock with Swagelok Ultra-Torr fittings, which are an o-ring seal that works on either metal or glass tubing. This is how I managed the metal to glass seal when needed. I didn't do much glasswork, though. My dad is a retired research chemist, and he did all of his own glasswork unless it was something really tricky.

If needed I could silver braze the Ultra-Torr into a larger object. It's just a bit of a puzzle to figure out what sizes of glass tubing mate with the various metal fitting sizes, because glass tubing sizing can be variable.