Add upb_Arena_SpaceAllocated function

Ref: protocolbuffers/protobuf#10291

Ruby types defined though native extensions should register
a function that report their memory footprint in bytes.

This feature is used by various memory profiling tools.

upb/arena.c

@@ -69,6 +69,19 @@ static upb_Arena* arena_findroot(upb_Arena* a) {
   return a;
 }
 
+size_t upb_Arena_SpaceAllocated(upb_Arena *arena) {
+  size_t memsize = 0;
+
+  mem_block* block = arena->freelist;
+
+  while (block) {
+    memsize += sizeof(mem_block) + block->size;
+    block = block->next;
+  }
+
+  return memsize;
+}
+
 static void upb_Arena_addblock(upb_Arena* a, upb_Arena* root, void* ptr,
                                size_t size) {
   mem_block* block = ptr;

upb/arena.h

@@ -72,6 +72,7 @@ void upb_Arena_Free(upb_Arena* a);
 bool upb_Arena_AddCleanup(upb_Arena* a, void* ud, upb_CleanupFunc* func);
 bool upb_Arena_Fuse(upb_Arena* a, upb_Arena* b);
 void* _upb_Arena_SlowMalloc(upb_Arena* a, size_t size);
+size_t upb_Arena_SpaceAllocated(upb_Arena *arena);
 
 UPB_INLINE upb_alloc* upb_Arena_Alloc(upb_Arena* a) { return (upb_alloc*)a; }