[exporter/signalfx] Fix memory leak on shutdown #27660

Job	Run time
collector-build	6m 49s
docker-build	7m 19s
supervisor-test	2m 12s
kubernetes-test-matrix (v1.26.0, receiver/k8sclusterreceiver)	6m 1s
kubernetes-test-matrix (v1.26.0, processor/k8sattributesprocessor)	7m 4s
kubernetes-test-matrix (v1.26.0, receiver/kubeletstatsreceiver)	5m 47s
kubernetes-test-matrix (v1.26.0, receiver/k8sobjectsreceiver)	4m 36s
kubernetes-test-matrix (v1.25.3, receiver/k8sclusterreceiver)	6m 7s
kubernetes-test-matrix (v1.25.3, processor/k8sattributesprocessor)	4m 42s
kubernetes-test-matrix (v1.25.3, receiver/kubeletstatsreceiver)	5m 43s
kubernetes-test-matrix (v1.25.3, receiver/k8sobjectsreceiver)	5m 23s
kubernetes-test-matrix (v1.24.7, receiver/k8sclusterreceiver)	7m 55s
kubernetes-test-matrix (v1.24.7, processor/k8sattributesprocessor)	4m 44s
kubernetes-test-matrix (v1.24.7, receiver/kubeletstatsreceiver)	5m 38s
kubernetes-test-matrix (v1.24.7, receiver/k8sobjectsreceiver)	4m 36s
kubernetes-test-matrix (v1.23.13, receiver/k8sclusterreceiver)	6m 18s
kubernetes-test-matrix (v1.23.13, processor/k8sattributesprocessor)	4m 49s
kubernetes-test-matrix (v1.23.13, receiver/kubeletstatsreceiver)	5m 41s
kubernetes-test-matrix (v1.23.13, receiver/k8sobjectsreceiver)	4m 40s
kubernetes-test	1s
	1h 46m 5s

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