[improve][client] Print consumer stats log if prefetched messages are not zero

[codelipenghui]

Motivation

Print consumer stats log if prefetched messages are not zero to help users know that the consumer just stop consuming messages from the receiver queue. Now, we only print the stats log if there is receive or ack throughput on the consumer.

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

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete