Metrics: Missed slots and leader - What are they?

Hey, I’m completely new to running a stake pool and cardano in general.

I have been running a stake pool on the testnet for a week or two now, trying to figure out how it works.
I am just using the regular cardano-node. Not cntools, cncli, gLiveView or anything like that.

I have also setup prometheus and grafana, but there are some metrics that I just don’t understand.

cardano_node_metrics_slotsMissedNum_int
cardano_node_metrics_Forge_node_not_leader_int
cardano_node_metrics_Forge_forge_about_to_lead_int

Can anyone explain what these metrics actually represent?
Are there any official documentation regarding metrics in general?