public void fillStackTrace() {
        // This method is public. If invoked concurrently, the this.stackTrace may be updated twice,
        // which is fine.

        this.stackTrace = Optional.of(threadLockStats.getStackTrace());
    }
Fill in the stack trace starting at the caller's stack frame.
zkfacade/src/main/java/com/yahoo/vespa/curator/stats/LockAttempt.java