In Request.finish(), the Runnable is calling toString on itself instead of the Request object:

    mainThread.post(new Runnable() {
        @Override
        public void run() {
            mEventLog.add(tag, threadId);
            mEventLog.finish(this.toString());
        }
    });

Should be Request.this.toString()

Also, it would be more robust and leak-resistant if this Runnable were a static nested class that held no reference to the outer Request instance.