Fix metric names
Description
As pointed out here, we can simplify the metric names by moving the provider type to a label.
Proposal
Metric names would change from things like:
ci_docker_machines_provider_actions_total{action="created"}
to
ci_machines_provider_actions_total{provider="docker",action="created"}
At the same time, we should properly namespace these metrics with gitlab_runner
instead of ci
.