[exporter/signalfx] Fix memory leak on shutdown #25930

Job	Run time
docker-build	6m 37s
collector-build	5m 47s
supervisor-test	55s
kubernetes-test-matrix (v1.26.0, receiver/k8sclusterreceiver)	4m 43s
kubernetes-test-matrix (v1.26.0, processor/k8sattributesprocessor)	3m 17s
kubernetes-test-matrix (v1.26.0, receiver/kubeletstatsreceiver)	4m 6s
kubernetes-test-matrix (v1.26.0, receiver/k8sobjectsreceiver)	3m 10s
kubernetes-test-matrix (v1.25.3, receiver/k8sclusterreceiver)	5m 36s
kubernetes-test-matrix (v1.25.3, processor/k8sattributesprocessor)	3m 23s
kubernetes-test-matrix (v1.25.3, receiver/kubeletstatsreceiver)	4m 44s
kubernetes-test-matrix (v1.25.3, receiver/k8sobjectsreceiver)	3m 14s
kubernetes-test-matrix (v1.24.7, receiver/k8sclusterreceiver)	4m 44s
kubernetes-test-matrix (v1.24.7, processor/k8sattributesprocessor)	3m 22s
kubernetes-test-matrix (v1.24.7, receiver/kubeletstatsreceiver)	4m 13s
kubernetes-test-matrix (v1.24.7, receiver/k8sobjectsreceiver)	3m 56s
kubernetes-test-matrix (v1.23.13, receiver/k8sclusterreceiver)	4m 43s
kubernetes-test-matrix (v1.23.13, processor/k8sattributesprocessor)	3m 37s
kubernetes-test-matrix (v1.23.13, receiver/kubeletstatsreceiver)	4m 17s
kubernetes-test-matrix (v1.23.13, receiver/k8sobjectsreceiver)	3m 17s
kubernetes-test	1s
	1h 17m 42s

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