移动 g_user_metric_label_count位置，避免循环引用 #1458

Job	Run time
ubuntu_clang (Release, OFF, ON)	2m 8s
ubuntu_clang_for_liburing (Release, OFF, ON)	2m 6s
ubuntu_clang10 (Release)	46s
ubuntu_clang_pb (Release, OFF, ON)	1m 52s
ubuntu_clang (Release, OFF, OFF)	2m 26s
ubuntu_clang_for_liburing (Debug, OFF, ON)	2m 3s
ubuntu_clang10 (Debug)	1m 9s
ubuntu_clang_pb (Release, OFF, OFF)	1m 28s
ubuntu_clang (Debug, OFF, ON)	2m 9s
ubuntu_clang_pb (Debug, OFF, ON)	1m 22s
ubuntu_clang (Debug, OFF, OFF)	2m 20s
ubuntu_clang_pb (Debug, OFF, OFF)	1m 53s
	21m 42s

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