Write documentation explaining how to enable metrics

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. I would like to see metrics for a query and I don't remember how to do that so I looked in the documentation but could not find any references to metrics, other than in the changelog. I ran find . -name "*.md" -exec grep -Hi metrics {} \;.

Describe the solution you'd like Documentation, preferably in the user guide, on how to enable metrics.

Describe alternatives you've considered None

Additional context None

[alamb]

This could perhaps start with showing EXPLAIN and then EXPLAIN ANALYZE to show the metrics and how to interpret them