fix: drop the first level of MultiIndex

[zhaoyongjie]

SUMMARY

Drop the first level of MultiIndex when there is only 1 metric in the query object

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

After

TESTING INSTRUCTIONS

CI

ADDITIONAL INFORMATION

• Has associated issue: closed Regression - Echarts Bar and Line chart label values changed unexpectectly when using Group By #19654
• 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

[villebro]

Very nice 👍 LGTM with one minor nit

[codecov]

Codecov Report

Merging #19716 (4c7ed13) into master (8865656) will decrease coverage by 0.03%.
The diff coverage is 53.57%.

❗ Current head 4c7ed13 differs from pull request most recent head 72ed22a. Consider uploading reports for the commit 72ed22a to get more accurate results

@@            Coverage Diff             @@
##           master   #19716      +/-   ##
==========================================
- Coverage   66.51%   66.47%   -0.04%     
==========================================
  Files        1684     1684              
  Lines       64560    64573      +13     
  Branches     6625     6630       +5     
==========================================
- Hits        42939    42927      -12     
- Misses      19926    19948      +22     
- Partials     1695     1698       +3     

Flag	Coverage Δ
hive	52.69% <66.66%> (+<0.01%)	⬆️
mysql	?
postgres	81.98% <100.00%> (-0.02%)	⬇️
presto	52.54% <66.66%> (+<0.01%)	⬆️
python	82.38% <100.00%> (-0.06%)	⬇️
sqlite	81.77% <100.00%> (+<0.01%)	⬆️
unit	47.75% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...superset-ui-core/src/query/types/PostProcessing.ts	100.00% <ø> (ø)
superset-frontend/src/views/components/SubMenu.tsx	85.96% <ø> (ø)
...perset-frontend/src/views/components/MenuRight.tsx	60.63% <31.57%> (-6.03%)	⬇️
...ui-chart-controls/src/operators/flattenOperator.ts	100.00% <100.00%> (ø)
...rset-ui-core/src/connection/SupersetClientClass.ts	100.00% <100.00%> (ø)
superset-frontend/src/views/CRUD/utils.tsx	64.80% <100.00%> (+0.28%)	⬆️
superset/utils/pandas_postprocessing/flatten.py	100.00% <100.00%> (ø)
superset/common/utils/dataframe_utils.py	85.71% <0.00%> (-7.15%)	⬇️
superset/db_engine_specs/mysql.py	93.97% <0.00%> (-3.62%)	⬇️
superset/reports/commands/log_prune.py	85.71% <0.00%> (-3.58%)	⬇️
... and 7 more

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