Update metric names to follow Prometheus's best practices

[Apollo-Roboto]

From this documentation: https://prometheus.io/docs/practices/naming/#metric-names

Should have a (single-word) application prefix relevant to the domain the metric belongs to.

I'm suggesting to add a minecraft_ prefix to every metric to follow Prometheus's best practices. Having this prefix would make it much easier to browse the exposed metrics.

For example:

player_score   ->   minecraft_player_score
damage_taken   ->   minecraft_player_damage_taken
mc_custom      ->   minecraft_custom