feat: per-db add metrics

[betodealmeida]

SUMMARY

This PR adds a method called get_metrics to the Database model and the base DB engine spec. It allows DB engine specs to return custom metrics for a given dataset, so that when a user adds a new dataset it can have more metrics than just COUNT(*) (the current behavior).

Currently no DB engine spec implements get_metrics, but in the future it could be leveraged by DB engine specs that have a semantic layer (like https://cube.dev/).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

Added tests.

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 #20990 (b07589d) into master (eec6e57) will decrease coverage by 0.82%.
The diff coverage is 67.64%.

❗ Current head b07589d differs from pull request most recent head 595e41d. Consider uploading reports for the commit 595e41d to get more accurate results

@@            Coverage Diff             @@
##           master   #20990      +/-   ##
==========================================
- Coverage   66.34%   65.51%   -0.83%     
==========================================
  Files        1767     1767              
  Lines       67284    67308      +24     
  Branches     7140     7144       +4     
==========================================
- Hits        44642    44100     -542     
- Misses      20815    21380     +565     
- Partials     1827     1828       +1     

Flag	Coverage Δ
hive	?
mysql	?
postgres	?
presto	53.05% <85.71%> (+0.01%)	⬆️
python	79.77% <95.23%> (-1.70%)	⬇️
sqlite	79.59% <95.23%> (+<0.01%)	⬆️
unit	50.49% <85.71%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
...ntend/src/explore/components/ExploreChartPanel.jsx	67.60% <0.00%> (-1.41%)	⬇️
.../src/views/CRUD/data/dataset/DatasetPage/index.tsx	0.00% <0.00%> (ø)
.../explore/components/ExploreViewContainer/index.jsx	52.45% <75.00%> (+0.26%)	⬆️
superset/common/query_context_processor.py	89.28% <80.00%> (-0.36%)	⬇️
superset/connectors/sqla/models.py	89.79% <100.00%> (-0.54%)	⬇️
superset/db_engine_specs/base.py	87.94% <100.00%> (-0.26%)	⬇️
superset/models/core.py	88.03% <100.00%> (-0.67%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/datasets/commands/update.py	25.00% <0.00%> (-69.05%)	⬇️
superset/datasets/commands/create.py	29.41% <0.00%> (-68.63%)	⬇️
... and 36 more

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