Move servlet adapters to an internal package to avoid duplicating classes when building OSGi bundles

[adessaigne]

This fixes #771

OSGi requires that any particular package is exposed by only one bundle (jar). With the Adapter class directly in the root io.prometheus.simpleclient it was copying all the other classes from this particular package and so copying the classes from simpleclient. I've chosen io.prometheus.simpleclient.internal as no package named internal is exported by default in OSGi. So, in OSGi environments, this class will only be visible by MetricsServlet and MetricsFilter

I've also included a fix of a typo that was preventing the adapter class to work as expected.

[fstab]

Thanks a lot for figuring this out. I am considering to start a 1.0 release branch. We could use this for breaking changes, like refactoring the module and package structure to make sure there's only one module for each package.

What do you think, if we start a 1.0 branch, how should it be structured?

[adessaigne]

Hi Fabian, thanks for having a look at this pull request.

Even if I moved around the Adapter class I don't think that it can be considered as breaking change as no one should call this class directly.

As for the 1.0 branch I'm not familiar enough with the codebase to make informed decision but I'll gladly help.

Note: I wrongly closed this pull request because I have big fingers on a small smartphone screen...

[fstab]

Thanks, I merged it.