Allow to specify time for log records

[Chi-teck]

That would be nice to allow users mock time for log records.

\Monolog\Logger::addRecord has an optional parameter for log timestamp. However, methods that implement Psr\Log\LoggerInterface do not have such a parameter.

I think the best way to fix this is passing optional $clock parameter to the logger constructor that implements PSR 20 interface.
Then addRecord could fetch timestamp from the clock service

[Seldaek]

Sounds reasonable to me if you want to send a PR :)