fix(dnd): make clicked dnd metrics unique

[villebro]

SUMMARY

When creating a metric using the click handler on DnD, we currently memoize an instance of adhocMetric that becomes the reference for all future clicked adhoc metrics. This causes all adhoc metrics that have been created by clicking to be mutated whenever we edit an adhoc metric that was also created by clicking.

To avoid unnecessary rerenders that would be a consequence of simply removing useMemo, we here instead initialize the droppedItem to an empty object, which then returns a fresh adhocMetric for each click.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

[codecov]

Codecov Report

Merging #16632 (7c3cd55) into master (519baa6) will increase coverage by 0.00%.
The diff coverage is 70.00%.

@@           Coverage Diff           @@
##           master   #16632   +/-   ##
=======================================
  Coverage   76.77%   76.77%           
=======================================
  Files        1003     1004    +1     
  Lines       53959    53965    +6     
  Branches     7330     7333    +3     
=======================================
+ Hits        41427    41432    +5     
- Misses      12293    12294    +1     
  Partials      239      239           

Flag	Coverage Δ
javascript	71.09% <70.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...ontrols/DndColumnSelectControl/DndMetricSelect.tsx	41.87% <57.14%> (+0.47%)	⬆️
...nd/src/explore/components/DatasourcePanel/types.ts	100.00% <100.00%> (ø)

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[kgabryje]

LGTM!

[stevenuray]

🏷️ 2021.36

[stevenuray]

🏷️ 2021.36

[rosemarie-chiu]

🏷 2021.34