Expose timestamp in collector.publish

This is a suggestion.

I am building a collector that parses log files (for instance web servers logs) to build metrics.
This allows for reconstruction of metrics in the past (eg. number of requests in each interval since last reading), so providing the timestamp when publishing the metrics is nice !

For the moment, it is explicitly not added to publish_counter, since derivative with past values will be broken when using data from the past.

There might be other limitations that I am not aware of ..

I'll be happy to have some feedback !