Add example for custom logger and LogLevel.toString

[RobertIndie]

Motivation

Currently, there is no example or documentation to show how to use the custom logger.

Modifications

• Add an example to show how to use the custom logger
• Add toString method to Pulsar.LogLevel to convert logLevel number to string

Verifying this change

Run node examples/custom_logger.js

Output:

[INFO][ClientConnection:189] [<none> -> pulsar://localhost:6650] Create ClientConnection, timeout=10000
[INFO][ConnectionPool:97] Created connection for pulsar://localhost:6650
[INFO][ClientConnection:379] [127.0.0.1:50422 -> 127.0.0.1:6650] Connected to broker
[INFO][HandlerBase:72] [persistent://public/default/my-topic, ] Getting connection from pool
[INFO][ProducerImpl:190] [persistent://public/default/my-topic, ] Created producer on broker [127.0.0.1:50422 -> 127.0.0.1:6650] 
[INFO][Client:87] Subscribing on Topic :persistent://public/default/my-topic
[INFO][HandlerBase:72] [persistent://public/default/my-topic, sub1, 0] Getting connection from pool
[INFO][ConsumerImpl:238] [persistent://public/default/my-topic, sub1, 0] Created consumer on broker [127.0.0.1:50422 -> 127.0.0.1:6650] 
my-message
[INFO][ProducerImpl:721] [persistent://public/default/my-topic, standalone-5-19] Closing producer for topic persistent://public/default/my-topic
[INFO][ProducerImpl:687] [persistent://public/default/my-topic, standalone-5-19] Closed producer 0
[INFO][ConsumerImpl:1097] [persistent://public/default/my-topic, sub1, 0] Closing consumer for topic persistent://public/default/my-topic
[INFO][ConsumerImpl:1083] [persistent://public/default/my-topic, sub1, 0] Closed consumer 0
[INFO][ClientImpl:517] Closing Pulsar client with 0 producers and 0 consumers
[INFO][ClientConnection:1584] [127.0.0.1:50422 -> 127.0.0.1:6650] Connection closed with ConnectError
[INFO][ClientConnection:267] [127.0.0.1:50422 -> 127.0.0.1:6650] Destroyed connection

Documentation

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• doc-not-needed
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• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)