carbonexporter: fix empty metric serialization (#30182) #47

Job	Run time
windows-unittest-matrix (receiver-0)	18m 5s
windows-unittest-matrix (receiver-1)	25m 29s
windows-unittest-matrix (processor)	17m 32s
windows-unittest-matrix (exporter)	23m 54s
windows-unittest-matrix (extension)	12m 43s
windows-unittest-matrix (internal)	11m 54s
windows-unittest-matrix (pkg)	12m 22s
windows-unittest-matrix (cmd)	33m 6s
windows-unittest-matrix (other)	20m 7s
windows-unittest	1s
	2h 55m 13s

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