You can enable reflection API (but must compile your server with it) - e.g. it exposes some well known end-point, which when asked can return you back the other end-points available (services), and then asking each end-point service can return each method - and then for each method - what it takes (assuming as "proto") and returns, also what kind is (one shot, bidi, etc.)

So not the same as GraphQL, as you are not asking about resources/objects, but rather inspecting what can be called and how.