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.
In this patch we add a registration mechanism for modules. To do so, a
module must defined the "promex_module" structure. The dump itself will be
based on 2 contexts. One for all the dump and another one for each metric
time-series. These contexts are used as restart points when the dump is
interrupted.
Modules must also implement 6 callback functions:
* start_metric_dump(): It is an optional callback function. If defined, it
is responsible to initialize the dump context use
as the first restart point.
* stop_metric_dump(): It is an optional callback function. If defined, it
is responsible to deinit the dump context.
* metric_info(): This one is mandatory. It returns the info about the
metric: name, type and flags and descrition.
* start_ts(): This one is mandatory, it initializes the context for a time
series for a given metric. This context is the second
restart point.
* next_ts(): This one is mandatory. It interates on time series for a
given metrics. It is also responsible to handle end of a
time series and deinit the context.
* fill_ts(): It fills info on the time series for a given metric : the
labels and the value.
In addition, a module must set its name and declare the number of metrics is
exposed.
