[CAE-1072] Hitless Upgrades (#3447)

* feat(hitless): Introduce handlers for hitless upgrades

This commit includes all the work on hitless upgrades with the addition
of:

- Pubsub Pool
- Examples
- Refactor of push
- Refactor of pool (using atomics for most things)
- Introducing of hooks in pool


---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

hitless/example_hooks.go

@@ -0,0 +1,100 @@
+package hitless
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/redis/go-redis/v9/internal"
+	"github.com/redis/go-redis/v9/internal/pool"
+	"github.com/redis/go-redis/v9/push"
+)
+
+// contextKey is a custom type for context keys to avoid collisions
+type contextKey string
+
+const (
+	startTimeKey contextKey = "notif_hitless_start_time"
+)
+
+// MetricsHook collects metrics about notification processing.
+type MetricsHook struct {
+	NotificationCounts map[string]int64
+	ProcessingTimes    map[string]time.Duration
+	ErrorCounts        map[string]int64
+	HandoffCounts      int64 // Total handoffs initiated
+	HandoffSuccesses   int64 // Successful handoffs
+	HandoffFailures    int64 // Failed handoffs
+}
+
+// NewMetricsHook creates a new metrics collection hook.
+func NewMetricsHook() *MetricsHook {
+	return &MetricsHook{
+		NotificationCounts: make(map[string]int64),
+		ProcessingTimes:    make(map[string]time.Duration),
+		ErrorCounts:        make(map[string]int64),
+	}
+}
+
+// PreHook records the start time for processing metrics.
+func (mh *MetricsHook) PreHook(ctx context.Context, notificationCtx push.NotificationHandlerContext, notificationType string, notification []interface{}) ([]interface{}, bool) {
+	mh.NotificationCounts[notificationType]++
+
+	// Log connection information if available
+	if conn, ok := notificationCtx.Conn.(*pool.Conn); ok {
+		internal.Logger.Printf(ctx, "hitless: metrics hook processing %s notification on conn[%d]", notificationType, conn.GetID())
+	}
+
+	// Store start time in context for duration calculation
+	startTime := time.Now()
+	_ = context.WithValue(ctx, startTimeKey, startTime) // Context not used further
+
+	return notification, true
+}
+
+// PostHook records processing completion and any errors.
+func (mh *MetricsHook) PostHook(ctx context.Context, notificationCtx push.NotificationHandlerContext, notificationType string, notification []interface{}, result error) {
+	// Calculate processing duration
+	if startTime, ok := ctx.Value(startTimeKey).(time.Time); ok {
+		duration := time.Since(startTime)
+		mh.ProcessingTimes[notificationType] = duration
+	}
+
+	// Record errors
+	if result != nil {
+		mh.ErrorCounts[notificationType]++
+
+		// Log error details with connection information
+		if conn, ok := notificationCtx.Conn.(*pool.Conn); ok {
+			internal.Logger.Printf(ctx, "hitless: metrics hook recorded error for %s notification on conn[%d]: %v", notificationType, conn.GetID(), result)
+		}
+	}
+}
+
+// GetMetrics returns a summary of collected metrics.
+func (mh *MetricsHook) GetMetrics() map[string]interface{} {
+	return map[string]interface{}{
+		"notification_counts": mh.NotificationCounts,
+		"processing_times":    mh.ProcessingTimes,
+		"error_counts":        mh.ErrorCounts,
+	}
+}
+
+// ExampleCircuitBreakerMonitor demonstrates how to monitor circuit breaker status
+func ExampleCircuitBreakerMonitor(poolHook *PoolHook) {
+	// Get circuit breaker statistics
+	stats := poolHook.GetCircuitBreakerStats()
+
+	for _, stat := range stats {
+		fmt.Printf("Circuit Breaker for %s:\n", stat.Endpoint)
+		fmt.Printf("  State: %s\n", stat.State)
+		fmt.Printf("  Failures: %d\n", stat.Failures)
+		fmt.Printf("  Last Failure: %v\n", stat.LastFailureTime)
+		fmt.Printf("  Last Success: %v\n", stat.LastSuccessTime)
+
+		// Alert if circuit breaker is open
+		if stat.State.String() == "open" {
+			fmt.Printf("  ⚠️  ALERT: Circuit breaker is OPEN for %s\n", stat.Endpoint)
+		}
+	}
+}