Add missing documentation for reactor latency metric
Add missing documentation for reactor latency metric

Change-Id: I93f18b3f820e2d72648a7142ef2fb344d7db289f
Reviewed-on: http://gerrit.cloudera.org:8080/8383
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Kudu Jenkins

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