[docs] Update stats.md

[ThaisAlvarenga]

Description: Added another way to write stats (stats="true") and reorganized the Metrics. I hope this is helpful.

Changes proposed:

• Included how to use stats='true'
• Reorganized Metrics so that the items are classified under a subheading according to how they appear in the actual UI display.

[dmarcos]

Thanks. I left some comments.