[exporter/signalfx] Fix goroutine leaks (#32781) #6

Job	Run time
windows-unittest-matrix (receiver-0)	15m 53s
windows-unittest-matrix (receiver-1)	16m 52s
windows-unittest-matrix (receiver-2)	17m 22s
windows-unittest-matrix (receiver-3)	16m 8s
windows-unittest-matrix (processor-0)	1m 45s
windows-unittest-matrix (processor-1)	12m 8s
windows-unittest-matrix (exporter-0)	9m 52s
windows-unittest-matrix (exporter-1)	18m 58s
windows-unittest-matrix (exporter-2)	10m 38s
windows-unittest-matrix (exporter-3)	13m 9s
windows-unittest-matrix (extension)	1m 43s
windows-unittest-matrix (connector)	9m 57s
windows-unittest-matrix (internal)	1m 54s
windows-unittest-matrix (pkg)	10m 33s
windows-unittest-matrix (cmd-0)	4m 47s
windows-unittest-matrix (cmd-1)	9m 34s
windows-unittest-matrix (other)	14m 39s
windows-unittest	0s
	3h 5m 52s

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