KUDU-1624. Fix data race in Trace::MetricsToJSON
KUDU-1624. Fix data race in Trace::MetricsToJSON

We were accessing child_traces_ without the appropriate lock.

rpc_stub-test failed 2/200 times before with this race, and passed
1000/1000 after the fix.

Change-Id: If72ccf3fc8e0d727f2c40b978889597e89ad891a
Reviewed-on: http://gerrit.cloudera.org:8080/5419
Tested-by: Kudu Jenkins
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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