metrics: extract mmodule_metrics in its own module

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
metrics: extract mclasses_metrics in its own nit module

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
metrics: refactor inheritance metrics

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
metrics: replace is_user_defined() by is_standard()

also fix the @FIXME

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
metrics: remove useless redef of Model from metrics_base

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
test: update test for nitmetrics

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>