Don't clear metrics before running handler

[istathar]

For some reason we thought we needed to clear the telemetry metrics before running the supplied handler function. This turned out to be unnecessary, because we are of course starting a new trace when we run a handler and that by definition starts with a clean slate. Calling clearMetrics was, however, resulting in us throwing away metrics being added by (for example) authorization machinery before the handler function was invoked. Fixed by removing the call.