Define events metrics group

[lambdanis]

See Tetragon metrics framework for more context.

• Define events metrics group (unconstrained) containing events metrics (currently initialized here). See how health metrics group is defined: https://github.com/cilium/tetragon/blob/main/pkg/metricsconfig/healthmetrics.go
• Move events metrics to a separate endpoint (breaking change)
• Disable events metrics by default (breaking change)
• Adjust how metrics docs are generated