[schema] Updating the base metric schema

[john-bodley]

We've noticed a number of anomalies in our database caused by ill-defined forms and/or table schema definitions. This PR resolves a number of issues related to the metrics and sql_metrics table including:

• Adds the missing legacy constraint to the sql_metrics table which was added in PR Adding YAML Import-Export for Datasources to CLI #3978 but didn't contain the corresponding migration. Note this requires reducing the size of the metric_name column for viability purposes.
• Reduces the size of metric_name column for the metrics table in the model which is inline with the previous migration.
• Ensures that the metric_name column is non-NULL.
• Ensures that the metrics.json and sql_metrics.expression columns are non-NULL.
• Deletes any orphaned records, i.e., those which are not associated with a datasource/table.

The uniqueness constraint was added to the metrics table in this migration.

Note this migration will fail if the metrics.metric_name, metrics.json, sql_metrics.metric_name, or sql_metrics.expression column is NULL. One must manually fix these records as programmatically trying to remedy these invalid records is difficult as the intent is unclear and the columns may function (from a query standpoint) if the underlying column expression is defined. The following queries determine which records are problematic:

SELECT 
    *
FROM 
    metrics
WHERE 
    metric_name IS NULL OR 
    json IS NULL

and

SELECT 
    *
FROM 
    sql_metrics
WHERE 
    metric_name IS NULL OR 
    expression IS NULL

Additionally this migration will truncate the sql_metrics.metric_name column from 512 to 255 characters. The following query determines which records are problematic:

SELECT
    *
FROM 
    sql_metrics
WHERE
    CHAR_LENGTH(metric_name) > 255

Finally this migration will fail if the sql_metrics table is corrupt in terms of the uniqueness constraint. One must manually consolidate duplicate or non-valid records given there's no way of programmatically removing invalid records. The following query determines whether there are duplicates:

SELECT 
    table_id, 
    metric_name,
    COUNT(1)
FROM 
    sql_metrics
GROUP BY 
    1, 2
HAVING 
    COUNT(1) > 1
ORDER BY 
    COUNT(1) DESC

Note this PR is gated by #5445 and #7084 which ensures that empty strings associated with form-data wont persist in the database and is necessary for ensuring that the relevant entries are non-NULL.

to: @fabianmenges @graceguo-supercat @michellethomas @mistercrunch @timifasubaa

[codecov-io]

Codecov Report

Merging #5453 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5453   +/-   ##
=======================================
  Coverage   64.92%   64.92%           
=======================================
  Files         428      428           
  Lines       20960    20960           
  Branches     2333     2333           
=======================================
  Hits        13608    13608           
  Misses       7228     7228           
  Partials      124      124

Impacted Files	Coverage Δ
superset/connectors/druid/models.py	82.49% <100%> (ø)	⬆️
superset/connectors/sqla/models.py	81.86% <100%> (ø)	⬆️
superset/connectors/base/models.py	89.89% <100%> (ø)	⬆️

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