Rural breakdown chart broken in World Bank example dashboard

[rjurney]

The Rural breakdown chart is broken because the metric 'sum__SP_RUR_TOTL' is missing.

Expected results

See a chart.

Actual results

See an error.

Screenshots

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How to reproduce the bug

1. Load examples
2. Go to Dashboard List
3. Select World Bank
4. See error

Environment

(please complete the following information):

• superset version: [master, 5cf454b4] and [release--0.31, 700cbf4f]
• python version: 3.6.7
• node.js version: v11.6.0
• npm version: 6.5.0

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

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