Do not rely on System.nanoTime for E2E metrics

The E2E metrics are calculated across JVMs where
the System.nanoTime cannot be trusted as source of
time as it is JVM-specific.

Use the System.currentTimeMillis and convert the
result into nanos for keeping the API compatibility
with the existing code.

Change-Id: I89fdcf7b5c8f50890be31476e2252ce796fbcf6a