other: Blossom metric fields for dataset API column selection

[john-bodley]

SUMMARY

Like for columns, this PR enables one to select specific metric fields in the dataset API.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI and tested in the Swagger UI, i.e., tested the GET /api/v1/dataset/{pk} RESTful API endpoint with the following query:

{
  "columns": [
    "columns.advanced_data_type",
    "metrics.changed_on",
    "metrics.created_on",
    "metrics.d3format",
    "metrics.description",
    "metrics.expression",
    "metrics.extra",
    "metrics.id",
    "metrics.metric_name",
    "metrics.metric_type",
    "metrics.verbose_name",
    "metrics.warning_text"
  ],
  "keys": [
    "none"
  ]
}

Also confirmed that,

{
  "columns": [
    "metrics"
  ],
  "keys": [
    "none"
  ]
}

still works for backwards compatibility and,

{
  "columns": [
    "metrics",
    "metrics.id"
  ],
  "keys": [
    "none"
  ]
}

only returns the metric ID, i.e., the metrics column is superseded by the metrics.id column, which is consistent from a deprecation standpoint.

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

[john-bodley]

Ensures backwards compatibility, i.e., non-breaking change.

[ktmud]

what's the harm of keeping them both?

[codecov]

Codecov Report

Merging #20732 (1d5473e) into master (bbaa4cc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #20732   +/-   ##
=======================================
  Coverage   66.36%   66.36%           
=======================================
  Files        1754     1754           
  Lines       66673    66673           
  Branches     7049     7049           
=======================================
  Hits        44249    44249           
  Misses      20627    20627           
  Partials     1797     1797           

Flag	Coverage Δ
hive	53.22% <ø> (ø)
javascript	51.93% <ø> (ø)
mysql	81.18% <ø> (ø)
postgres	81.26% <ø> (ø)
presto	53.08% <ø> (ø)
python	81.70% <ø> (ø)
sqlite	81.07% <ø> (ø)
unit	50.44% <ø> (ø)

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Impacted Files	Coverage Δ
superset/datasets/api.py	88.52% <ø> (ø)

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