docs: Update example code, add log initialization configurations

[geniusjoe]

Master Issue: #1234

Motivation

The log/slog package, introduced in Go 1.21 has become the standard library for structured logging, which provides support for redirecting logs to files and enabling log rotation.
Additionally, since pulsar default logger uses log.NewLoggerWithLogrus(logrus.StandardLogger()), logs cannot be output to the files when users haven't set up logrus explicit configuration. Therefore, maybe we should use slog and give a complete log settings in our examples to enhance the out-of-the-box experience for users.

Modifications

Add log initialization configurations in example files.

Verifying this change

• Make sure that the change passes the CI checks.
  This change is a trivial rework without any test coverage.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[Copilot]

Pull request overview

This PR updates example files to use Go's standard log/slog package (introduced in Go 1.21) for structured logging, replacing the previous basic logging approach. It adds comprehensive logging configuration with file output and rotation capabilities via the lumberjack library to improve the out-of-the-box experience for users.

Key Changes:

• Adds gopkg.in/natefinch/lumberjack.v2 dependency for log rotation support
• Updates all four example files (producer, consumer, reader, consumer-listener) with complete slog logger initialization
• Replaces log.Fatal() and fmt.Printf() calls with structured logger.Error() and logger.Info() calls

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
go.mod	Adds lumberjack.v2 dependency for log rotation functionality
go.sum	Adds checksums for lumberjack.v2 dependency
examples/reader/reader.go	Implements slog logger with file rotation and updates all logging calls to use structured logging
examples/producer/producer.go	Implements slog logger with file rotation and updates all logging calls to use structured logging
examples/consumer/consumer.go	Implements slog logger with file rotation and updates all logging calls to use structured logging
examples/consumer-listener/consumer-listener.go	Implements slog logger with file rotation and updates all logging calls to use structured logging

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