I think we need to add a description to Metrics.md.
Other than that, I think the patch is good (I don't see any related unit tests for this and the left checkstyle would break the style of the class).
Andrew Wang, Arpit Agarwal, do you guys think this is a useful addition?
We found that the heartbeats were reporting as running smoothly but the block report processing was actually getting stuck because of the disk and delaying the heartbeats which wasn't easy to monitor. Actually, we are planning to open a separate JIRA to move some of the disk related checks to a separate thread.