stats: fix LuaJit breaking pairs __gc

In some cases LuaJit optimizes using gc_observer table to handle pairs
object gc. It had lead to incorrect behavior (ignoring some pairs
interrupted with break in stats) and tests fail in some cases
(for example, if you run only stats unit tests).

Part of #224

crud/stats/init.lua

@@ -117,6 +117,12 @@ local function setmt__gc(t, mt)
     return setmetatable(t, mt)
 end
 
+-- If jit will be enabled here, gc_observer usage
+-- may be optimized so our __gc hack will not work.
+local function keep_observer_alive(gc_observer) --luacheck: ignore
+end
+jit.off(keep_observer_alive)
+
 local function wrap_pairs_gen(build_latency, space_name, op, gen, param, state)
     local total_latency = build_latency
 
@@ -139,7 +145,7 @@ local function wrap_pairs_gen(build_latency, space_name, op, gen, param, state)
     local wrapped_gen = function(param, state)
         -- Mess with gc_observer so its lifespan will
         -- be the same as wrapped_gen() function.
-        gc_observer[1] = state
+        keep_observer_alive(gc_observer)
 
         local start_time = clock.monotonic()