MEDIUM: promex: Dump metrics of registered modules with a way to filter them

MEDIUM: promex: Dump metrics of registered modules with a way to filter them

This patch adds a dump loop on the registered modules. It is very similar to
other dump loops. When a module registered, a implicit scope is created with
the module's name. It means a module name must be unique. It also means,
metrics dump of modules can be filtered via the "scope" parameter.