官方issue并不承认这是一个问题，参考：
https://github.com/prometheus/prometheus/issues/5727
https://github.com/prometheus/prometheus/issues/4324
有人认为是磁盘读写慢导致的，参考
https://groups.google.com/forum/#!searchin/prometheus-users/prometheus$20hang$20$20component$3Dtsdb$20msg$3D%22found$20healthy$20block%22|sort:date/prometheus-users/SW9sy_-DSxA/BsgasOiTBAAJ