{
 timestamp: 1558475434,
 request_id: "83e21b28-7c12-11e9-8f9e-2a86e4085a59",
 user_id: "example_user_id_1",
 traits: {
  this: "is",
  dynamic: "json",
  as: ["defined","by","the", "client"]
 }
}

SELECT DISTINCT trait, min_by(json_extract(traits, concat('$.', cast(trait AS varchar))), received_at) AS value
FROM TABLE
CROSS JOIN UNNEST(regexp_extract_all(traits,'"([^"]+)"s*:s*("[^"]+"|[^,{}]+)', 1)) AS t(trait)
WHERE json_extract(traits, concat('$.', cast(trait AS varchar))) IS NOT NULL OR json_size(traits, concat('$.', cast(trait AS varchar))) <> 0
GROUP BY  trait