MetricRegistryMBean is not thread safe due to usage of histogramNames instance of HashMap class. Changing HashMap to ConcurrentHashMap will not help a lot (likely) because method MetricUtils.histogramBucketNames() uses just put method (putIfAbsent will help I believe).
OpenCensusExporterSpi uses the same MetricUtils.histogramBucketNames() method. But it isn't issue for this exporter because it is single threaded.
Also MetricUtils.histogramBucketNames() method is responsible for histogram bucket's name representation. I believe that it is responsibility of metric exporter and this method should be removed from MetricUtils.