introduce `obj_ary_extracapa`.

Introduce a new debug counter `obj_ary_extracapa` which counts
arrays which are `len < capa`.

array.c

@@ -786,6 +786,8 @@ void
 rb_ary_free(VALUE ary)
 {
     if (ARY_OWNS_HEAP_P(ary)) {
+        (void)RB_DEBUG_COUNTER_INC_IF(obj_ary_extracapa, ARY_HEAP_CAPA(ary) > RARRAY_LEN(ary));
+
         if (RARRAY_TRANSIENT_P(ary)) {
             RB_DEBUG_COUNTER_INC(obj_ary_transient);
         }

debug_counter.h

@@ -206,6 +206,7 @@ RB_DEBUG_COUNTER(obj_str_fstr)
 RB_DEBUG_COUNTER(obj_ary_embed)
 RB_DEBUG_COUNTER(obj_ary_transient)
 RB_DEBUG_COUNTER(obj_ary_ptr)
+RB_DEBUG_COUNTER(obj_ary_extracapa)
 /*
   ary_shared_create: shared ary by Array#dup and so on.
   ary_shared: finished in shard.