Emit message which merged user-supplied argument in log_handler

When using PublishErrorsHandler, it will missing user-supplied arguments.
For example, do LOG.info("blabla %s", "foo"), the payload only contains
"blabla %s", but we expect it like "blabla foo". This patch fix it.

Change-Id: I91289cc4a60f5dab89bca852e6f52b4b83831e47
Closes-bug: #1278291
Merge "Emit message which merged user-supplied argument in log_handler"