metrics: add Counter#print_elements

Signed-off-by: Jean Privat <jean@pryen.org>
metrics: use print_elements in static_type_metrics

Signed-off-by: Jean Privat <jean@pryen.org>
metrics: add ast_metrics

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update sav/nitmetrics

Signed-off-by: Jean Privat <jean@pryen.org>
Merge branch 'ast_metrics'