[Lens] Update name of "Columns" to something more descriptive

[ghudgins]

Describe the feature:
Update the Lens Table "Columns" configuration name to something more descriptive - "Split metrics by"

Describe a specific use case for the feature:
When I am configuring a Lens table
I need to understand the difference between what "Columns" does vs. what "Metrics" does as they are both columns
So I can avoid configuring rows & columns config on my table and being confused by the result

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[ghudgins]

Other ideas we considered (in lieu of "Split metrics by")

• 🚀 Split metrics by
• Split by
• Pivot by
• Pivot metrics by
• Transpose metrics by
• Transpose