Debt is the canary in the coal mine for market bubbles. Housing debt fueled the global financial crisis. Corporate debt led to dotcom bust. Now, the tech companies driving today's bull market are quietly levering up, sometimes through private lenders that make their debt less visible to shareholders.Why it matters: That debt — and how it is getting structured — is "almost an acknowledgement that this is getting out of hand," Dario Perkins, managing director of global macro at TS Lombard, tells A...