fix: Ensure metrics is an array in Mixed Chart

[kgabryje]

SUMMARY

In a Mixed Chart metadata from examples db (Average and Sum Trends), metrics and metrics_b is a single value instead of an array. This PR fixes that + adds failsafe in Mixed Chart metrics array spreading

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Add Average and Sum Trends chart to a dashboard, make sure it loads correctly

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[michael-s-molina]

LGTM

[codecov]

Codecov Report

Merging #24643 (b949333) into master (c53b249) will increase coverage by 0.08%.
The diff coverage is n/a.

❗ Current head b949333 differs from pull request most recent head f499356. Consider uploading reports for the commit f499356 to get more accurate results

@@            Coverage Diff             @@
##           master   #24643      +/-   ##
==========================================
+ Coverage   68.97%   69.06%   +0.08%     
==========================================
  Files        1907     1907              
  Lines       74153    74153              
  Branches     8182     8182              
==========================================
+ Hits        51148    51213      +65     
+ Misses      20882    20817      -65     
  Partials     2123     2123              

Flag	Coverage Δ
hive	54.14% <ø> (?)
postgres	79.56% <ø> (ø)
presto	54.04% <ø> (ø)
python	83.50% <ø> (+0.18%)	⬆️
sqlite	78.12% <ø> (ø)
unit	54.83% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...hart-echarts/src/MixedTimeseries/transformProps.ts	0.78% <ø> (ø)
superset/examples/birth_names.py	70.00% <ø> (ø)

... and 4 files with indirect coverage changes

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