insights: change cluster_execution_insights priority column from FLOAT8 to STRING

[ericharmeling]

The priority column type is FLOAT8:

root@localhost:26257/defaultdb> show columns from crdb_internal.cluster_execution_insights;
       column_name      | data_type | is_nullable | column_default | generation_expression | indices | is_hidden
------------------------+-----------+-------------+----------------+-----------------------+---------+------------
  session_id            | STRING    |      f      | NULL           |                       | {}      |     f
  txn_id                | UUID      |      f      | NULL           |                       | {}      |     f
  txn_fingerprint_id    | BYTES     |      f      | NULL           |                       | {}      |     f
  stmt_id               | STRING    |      f      | NULL           |                       | {}      |     f
  stmt_fingerprint_id   | BYTES     |      f      | NULL           |                       | {}      |     f
  problems              | STRING[]  |      f      | NULL           |                       | {}      |     f
  query                 | STRING    |      f      | NULL           |                       | {}      |     f
  status                | STRING    |      f      | NULL           |                       | {}      |     f
  start_time            | TIMESTAMP |      f      | NULL           |                       | {}      |     f
  end_time              | TIMESTAMP |      f      | NULL           |                       | {}      |     f
  full_scan             | BOOL      |      f      | NULL           |                       | {}      |     f
  user_name             | STRING    |      f      | NULL           |                       | {}      |     f
  app_name              | STRING    |      f      | NULL           |                       | {}      |     f
  database_name         | STRING    |      f      | NULL           |                       | {}      |     f
  plan_gist             | STRING    |      f      | NULL           |                       | {}      |     f
  rows_read             | INT8      |      f      | NULL           |                       | {}      |     f
  rows_written          | INT8      |      f      | NULL           |                       | {}      |     f
  priority              | FLOAT8    |      f      | NULL           |                       | {}      |     f
  retries               | INT8      |      f      | NULL           |                       | {}      |     f
  last_retry_reason     | STRING    |      t      | NULL           |                       | {}      |     f
  exec_node_ids         | INT8[]    |      f      | NULL           |                       | {}      |     f
  contention            | INTERVAL  |      t      | NULL           |                       | {}      |     f
  index_recommendations | STRING[]  |      f      | NULL           |                       | {}      |     f

This column should probably be a STRING (ENUM, really), since the priority values are LOW, NORMAL, and HIGH.

Jira issue: CRDB-18975

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