Refactor confusing function/variable/parameter names

[jaypha]

The function 'get_metrics' is used in different places, and do different things. They should be renamed to reduce potential confusion.

• [ ] metric\manager::get_metrics() -> get_metric_types().
• [ ] cltr_database\collector::get_metrics() -> get_saved_metric_data().
• [ ] metric_item::metric -> metrictype.
• [ ] $item in metric_item::__construct -> $metrictype