fix(examples): missing expressions in birth_names

[villebro]

SUMMARY

The metadata fetch call in load-examples erases the added num_california and sum__num expressions. By fetching before adding the expressions they are not erased prior to the commit.

BEFORE

AFTER

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #11141 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #11141      +/-   ##
==========================================
+ Coverage   65.80%   65.86%   +0.05%     
==========================================
  Files         826      826              
  Lines       39019    39032      +13     
  Branches     3667     3677      +10     
==========================================
+ Hits        25678    25709      +31     
+ Misses      13231    13213      -18     
  Partials      110      110              

Flag	Coverage Δ
#cypress	56.21% <ø> (-0.13%)	⬇️
#javascript	62.20% <ø> (-0.02%)	⬇️
#python	61.35% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
superset/examples/birth_names.py	100.00% <100.00%> (ø)
.../explore/components/controls/DatasourceControl.jsx	73.07% <0.00%> (-11.54%)	⬇️
.../src/explore/components/AdhocMetricEditPopover.jsx	75.64% <0.00%> (-5.13%)	⬇️
...-frontend/src/datasource/ChangeDatasourceModal.tsx	73.21% <0.00%> (-1.79%)	⬇️
...perset-frontend/src/views/CRUD/chart/ChartList.tsx	82.35% <0.00%> (-0.85%)	⬇️
...src/explore/components/controls/MetricsControl.jsx	89.69% <0.00%> (-0.61%)	⬇️
superset/views/core.py	74.30% <0.00%> (+0.24%)	⬆️
...erset-frontend/src/datasource/DatasourceEditor.jsx	71.36% <0.00%> (+0.45%)	⬆️
superset/models/core.py	88.39% <0.00%> (+0.82%)	⬆️
...t-frontend/src/dashboard/actions/dashboardState.js	51.63% <0.00%> (+1.30%)	⬆️
... and 3 more

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