[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>

logging/logging.go

@@ -0,0 +1,121 @@
+// Package logging provides logging level constants and utilities for the go-redis library.
+// This package centralizes logging configuration to ensure consistency across all components.
+package logging
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/redis/go-redis/v9/internal"
+)
+
+// LogLevel represents the logging level
+type LogLevel int
+
+// Log level constants for the entire go-redis library
+const (
+	LogLevelError LogLevel = iota // 0 - errors only
+	LogLevelWarn                  // 1 - warnings and errors
+	LogLevelInfo                  // 2 - info, warnings, and errors
+	LogLevelDebug                 // 3 - debug, info, warnings, and errors
+)
+
+// String returns the string representation of the log level
+func (l LogLevel) String() string {
+	switch l {
+	case LogLevelError:
+		return "ERROR"
+	case LogLevelWarn:
+		return "WARN"
+	case LogLevelInfo:
+		return "INFO"
+	case LogLevelDebug:
+		return "DEBUG"
+	default:
+		return "UNKNOWN"
+	}
+}
+
+// IsValid returns true if the log level is valid
+func (l LogLevel) IsValid() bool {
+	return l >= LogLevelError && l <= LogLevelDebug
+}
+
+func (l LogLevel) WarnOrAbove() bool {
+	return l >= LogLevelWarn
+}
+
+func (l LogLevel) InfoOrAbove() bool {
+	return l >= LogLevelInfo
+}
+
+func (l LogLevel) DebugOrAbove() bool {
+	return l >= LogLevelDebug
+}
+
+// VoidLogger is a logger that does nothing.
+// Used to disable logging and thus speed up the library.
+type VoidLogger struct{}
+
+func (v *VoidLogger) Printf(_ context.Context, _ string, _ ...interface{}) {
+	// do nothing
+}
+
+// Disable disables logging by setting the internal logger to a void logger.
+// This can be used to speed up the library if logging is not needed.
+// It will override any custom logger that was set before and set the VoidLogger.
+func Disable() {
+	internal.Logger = &VoidLogger{}
+}
+
+// Enable enables logging by setting the internal logger to the default logger.
+// This is the default behavior.
+// You can use redis.SetLogger to set a custom logger.
+//
+// NOTE: This function is not thread-safe.
+// It will override any custom logger that was set before and set the DefaultLogger.
+func Enable() {
+	internal.Logger = internal.NewDefaultLogger()
+}
+
+// NewBlacklistLogger returns a new logger that filters out messages containing any of the substrings.
+// This can be used to filter out messages containing sensitive information.
+func NewBlacklistLogger(substr []string) internal.Logging {
+	l := internal.NewDefaultLogger()
+	return &filterLogger{logger: l, substr: substr, blacklist: true}
+}
+
+// NewWhitelistLogger returns a new logger that only logs messages containing any of the substrings.
+// This can be used to only log messages related to specific commands or patterns.
+func NewWhitelistLogger(substr []string) internal.Logging {
+	l := internal.NewDefaultLogger()
+	return &filterLogger{logger: l, substr: substr, blacklist: false}
+}
+
+type filterLogger struct {
+	logger    internal.Logging
+	blacklist bool
+	substr    []string
+}
+
+func (l *filterLogger) Printf(ctx context.Context, format string, v ...interface{}) {
+	msg := fmt.Sprintf(format, v...)
+	found := false
+	for _, substr := range l.substr {
+		if strings.Contains(msg, substr) {
+			found = true
+			if l.blacklist {
+				return
+			}
+		}
+	}
+	// whitelist, only log if one of the substrings is present
+	if !l.blacklist && !found {
+		return
+	}
+	if l.logger != nil {
+		l.logger.Printf(ctx, format, v...)
+		return
+	}
+}