KUDU-241: initialize thread metrics
KUDU-241: initialize thread metrics

Whoops. C 101.

Made them uint64_t while I was at it; that seems to be more consistent with
the rest of Kudu.

I couldn't repro the "mystery thread" phenomenon seen by Todd, but I'm
hoping that Alex's commit db177a3 will prevent it from happening again.

Change-Id: I78c409a339fd9c02660c6c5dba940c2c25949bd6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2374
Tested-by: jenkins
Reviewed-by: Colin McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@cloudera.com>

InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu