I think you're going to have to dig into some really old stuff to find specific answers to this. I've been trying to remember the details, and some of it has come back, but not all of it.
I did this a few times when our shop was making scientific apparatus for a local university lab. Instructions came with the job. Roughly, it was this: Etch the glass lightly with (I think) nitric acid. The acid etch isn't absolutely necessary but it's generally used on jobs that require a good hermetic seal, which is what this was.
Then you "tin," or actually metallize, the glass with metal powder and a borate flux, using a torch flame and heating the glass hot enough to get the metal to flow. IIRC, the powder we used was either silver or tin.
Once the glass is metallized you can solder it any way you want. The specs on our job called for ordinary tin/lead solder (this was around 1974). Of course you have to be careful about heating the glass evenly, but the job itself was pretty easy. I did a number of them with no previous experience, and they all passed a pressure test.
Ed Huntress