Memory leak on macOS

[charlie0129]

How to reproduce:

• Build node_exporter from master branch
• Run node_exporter
• Hit it using http benchmark tool or just while true; do curl 127.0.0.1:9100/metrics >/dev/null; done
• Watch memory usage goes up.

[charlie0129]

I have identified the memory leak comes from #3206

I may open a PR to fix it