fix(sqla): apply jinja to metrics

[villebro]

SUMMARY

Currently template kwargs like from_dttm aren't available in metrics expressions. This fixes the issue and makes the expression processing code DRYer by calling the same code path in all places where the SQLA model handles adhoc expressions.

AFTER

Now the jinja in the adhoc metric that references from_dttm renders successfully:

BEFORE

Previously trying to reference the from_dttm value would cause rendering to fail:

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue: closes {{ from_dttm }} and {{ to_dttm }} Jinja special variables are not rendered in Custom SQL metrics #19564
• 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 #19565 (5026067) into master (e1ef2ba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #19565   +/-   ##
=======================================
  Coverage   66.59%   66.59%           
=======================================
  Files        1682     1682           
  Lines       64321    64317    -4     
  Branches     6561     6561           
=======================================
- Hits        42837    42835    -2     
+ Misses      19783    19781    -2     
  Partials     1701     1701           

Flag	Coverage Δ
hive	52.69% <100.00%> (-0.03%)	⬇️
mysql	81.91% <100.00%> (+<0.01%)	⬆️
postgres	81.95% <100.00%> (+<0.01%)	⬆️
presto	52.54% <100.00%> (-0.03%)	⬇️
python	82.39% <100.00%> (+<0.01%)	⬆️
sqlite	81.73% <100.00%> (+<0.01%)	⬆️
unit	47.22% <6.25%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	89.45% <100.00%> (+0.14%)	⬆️

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[zhaoyongjie]

LGTM. Test in my local, and behavior is expected.