[exporter/signalfx] Fix memory leaks #29127

Job	Run time
collector-build	4m 59s
docker-build	5m 30s
supervisor-test	52s
kubernetes-test-matrix (v1.26.0, receiver/k8sclusterreceiver)	5m 56s
kubernetes-test-matrix (v1.26.0, processor/k8sattributesprocessor)	4m 35s
kubernetes-test-matrix (v1.26.0, receiver/kubeletstatsreceiver)	4m 18s
kubernetes-test-matrix (v1.26.0, receiver/k8sobjectsreceiver)	3m 14s
kubernetes-test-matrix (v1.25.3, receiver/k8sclusterreceiver)	5m 47s
kubernetes-test-matrix (v1.25.3, processor/k8sattributesprocessor)	5m 6s
kubernetes-test-matrix (v1.25.3, receiver/kubeletstatsreceiver)	4m 29s
kubernetes-test-matrix (v1.25.3, receiver/k8sobjectsreceiver)	3m 21s
kubernetes-test-matrix (v1.24.7, receiver/k8sclusterreceiver)	4m 46s
kubernetes-test-matrix (v1.24.7, processor/k8sattributesprocessor)	4m 30s
kubernetes-test-matrix (v1.24.7, receiver/kubeletstatsreceiver)	5m 9s
kubernetes-test-matrix (v1.24.7, receiver/k8sobjectsreceiver)	3m 36s
kubernetes-test-matrix (v1.23.13, receiver/k8sclusterreceiver)	4m 48s
kubernetes-test-matrix (v1.23.13, processor/k8sattributesprocessor)	4m 46s
kubernetes-test-matrix (v1.23.13, receiver/kubeletstatsreceiver)	5m 11s
kubernetes-test-matrix (v1.23.13, receiver/k8sobjectsreceiver)	3m 36s
kubernetes-test	2s
	1h 24m 31s

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