metrics registration lifecycle

Ensure proper metrics registration and lifecycle management to maintain observability system stability. Handle metric registration/unregistration correctly during configuration changes and avoid common pitfalls that can cause panics or data loss.

Key practices:

1. Use AlreadyRegisteredError pattern for shared metrics: When multiple components share the same metrics, handle registration conflicts gracefully by retrieving existing metrics from the registry instead of creating new ones.

2. Manage registration during config reloads: Properly unregister metrics before re-registering to avoid panics, but be aware this causes counter resets. Consider keeping metrics registered and reusing them when possible.

3. Register all required collectors: When using custom registries, ensure all necessary collectors (GoCollector, ProcessCollector) are registered, not just the default ones.

Example of proper AlreadyRegisteredError handling:

func newDiscovererMetrics(reg prometheus.Registerer) *zookeeperMetrics {
    m := &zookeeperMetrics{
        failureCounter: prometheus.NewCounter(prometheus.CounterOpts{
            Name: "zookeeper_failures_total",
            Help: "The total number of ZooKeeper failures.",
        }),
    }
    
    // Handle shared metrics between components
    if err := reg.Register(m.failureCounter); err != nil {
        if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
            // Use the existing metric instead of the new one
            m.failureCounter = are.ExistingCollector.(prometheus.Counter)
        } else {
            // Handle other registration errors
            return nil
        }
    }
    return m
}

This prevents registration panics while ensuring metrics remain functional across component lifecycles and configuration changes.