chore(fab): Expose the SqlMetricInlineView extra column listed in the legacy FAB API

[john-bodley]

SUMMARY

As mentioned in Slack we (Airbnb) are experiencing performance issues with the Superset v1 API—in relation to a large dataset—due to an inefficiency in how the underlying FAB API fetch metadata. We're restricted to using the legacy FAB API however not all columns (for our needs) are exposed and thus this PR exposes the metric extra column to the list of columns which are included when reading/listing the metric metadata.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Ran http://127.0.0.1:8000/sqlmetricinlineview/api/read?flt_0_table_id=1 and confirmed that the extra column was included.

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 #21118 (166e190) into master (55304b0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #21118   +/-   ##
=======================================
  Coverage   66.30%   66.30%           
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  Files        1772     1772           
  Lines       67612    67612           
  Branches     7204     7204           
=======================================
  Hits        44832    44832           
  Misses      20938    20938           
  Partials     1842     1842           

Flag	Coverage Δ
hive	53.17% <100.00%> (ø)
mysql	80.96% <100.00%> (ø)
postgres	81.02% <100.00%> (ø)
presto	53.07% <100.00%> (ø)
python	81.50% <100.00%> (ø)
sqlite	79.62% <100.00%> (ø)
unit	50.74% <100.00%> (ø)

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

Impacted Files	Coverage Δ
superset/connectors/sqla/views.py	87.30% <100.00%> (ø)

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