Convert metricunit DB enum to varchar to eliminate migration overhead

[shangyian]

Problem

The unit column on metricmetadata is stored as a PostgreSQL native ENUM type (metricunit). This means that every time a new unit is added to MetricUnit in Python, it requires a database migration just to register the new value with the DB, even though the application logic doesn't otherwise change.

This has already happened multiple times (eg most recently in #1869). The downgrade path is also problematic: Postgres doesn't support removing enum values directly, so downgrade has to drop and recreate the entire enum type.

Proposed Solution

Change the unit column from Enum(MetricUnit) to VARCHAR. The python-side MetricUnit enum stays as-is for validation, but we stop using Postgres's native enum type to store it:

  unit: Optional[MetricUnit] = Field(
      sa_column=SqlaColumn(String),
      default=MetricUnit.UNKNOWN,
  )

We can do a one-time db migration to convert the column type:

ALTER TABLE metricmetadata ALTER COLUMN unit TYPE VARCHAR USING unit::text

After this change, adding new metric units requires zero DB migrations.

[shangyian]

@jlhester This one might be nice to start with, in case you have bandwidth!