On the surface, this seems like a silly question, but why do we have a metal-gear-solid-series tag? Please bear in mind that there is the assumption that users reading this question will have a good understanding of the series, as there is a lot of potential spoiler, below.
Metal Gear Solid is a title given to a majority of the games, in the Metal Gear series, but not all of them. Please keep in mind that I have played since Metal Gear Solid, and my knowledge of the predecessors mostly comes from research undergone through University. That said, I am confident enough to say they are still considered canonical predecessors to the games, as there is constant reference to them, in the games I have played.
The original games were called 'Metal Gear', and the series is still referred to as the 'Metal Gear Series'. 'Metal Gear Solid' was first coined with the first PlayStation title, where players play as Solid Snake. Solid Snake has since been a main protagonist, but not in every game. For example, in 'Metal Gear Solid 3: Snake Eater', you play as the Snake from the Metal Gear period.
Here are a few questions that, in context, include the original games/ naming convention:
What will I miss if I start playing with MGSV? - OP asks what they will miss, not having played the original games. The accepted answer provides a short summary of many of the titles, including several 'Metal Gear' games. Furthermore, MGSV ties in to elements of both Metal Gear and Metal Gear 2, from all I have seen.
What order do the games take place, chronologically? - OP asks about the chronology of the series. A good question, IMO, given that the series does not release in order of events. The accepted answer gives the in-game year of each game, placing Metal Gear and Metal Gear 2 around the middle of the list.
So why do we have a metal-gear-solid-series tag? Perhaps it was first implemented by a user who believed Metal Gear Solid to be the first title, and has since just been used since its there? Perhaps I am missing something, completely, here.