Seperate content format and sending in PushGatewayPublisher

[vipsy]

Added public methods to get formatted content suitable for PushGateway
but without sending it. This can be helpful in cases where you have
some middleware where you want to send this data which eventually
pushes to your pushgateway.

[vipsy]

@cdsap the commit message above explains why I need just the data without publishing them.

[vipsy]

I realised without separating formatter, I wont be able to use it in my custom publisher.
So created a formatter class. It's in the same package right now.
Feel free to suggest an appropriate package for it.

[cdsap]

hi @vipsy, thank you very much for the contribution.
I see ok the changes.
Related to the package name, it could be:
com.cdsap.talaiot.formatter
or
com.cdsap.talaiot.publisher.pushgateway including in this new package the publisher and the formater

[vipsy]

hi @cdsap I have moved them to com.cdsap.talaiot.publisher.pushgateway package to be consistent with graphpublisher and timeline

[cdsap]

perfect, merging. Next release will be published next week. After closing issue #93

Thank you!