This is not a general purpose profiling solution, but these JSON logs are already being
generated and stored, so this is worth adding.

Some administrators are very keen to know how long authentication
takes, particularly due to long replication transactions in other
processes.

This complements a similar patch set to log the transaction duration.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>