Potential race condition in Singleton implementation of SharedCacheUploaderMetrics, CleanerMetrics, ClientSCMMetrics

  public static ClientSCMMetrics getInstance() {
    ClientSCMMetrics topMetrics = Singleton.INSTANCE.impl;
    if (topMetrics == null) {
      throw new IllegalStateException(

getInstance() doesn't hold lock on Singleton.this
This may result in IllegalStateException being thrown prematurely.

ctrezzo reported that SharedCacheUploaderMetrics has also same kind of race condition.