MINOR: stats: hide px/sv/li fields in applet struct

Use an opaque pointer to store proxy instance. Regroup server/listener
as a single opaque pointer. This has the benefit to render the structure
more evolutive to support statistics on other types of objects in the
future.

This patch is needed to extend stat support for components other than
proxies objects.

The prometheus module has been adapted for these changes.
3 files changed