As part of the viewless timeseries / eliminate dual timeseries namespace track (SPM-3969 / SPM-3830) we added a new oplog 'c' (command) entry type that transforms a timeseries collection from viewful format ("myts" view + "system.buckets.myts" collection) to viewless format ("myts" timeseries collection) or vice versa.

This oplog entry is generated during a setFCV upgrade/downgrade across versions 8.x ↔ 9.0.

The format of the new oplog entry is as follows:

*UPGRADE FROM 8.x TO 9.0*

{
  op: "c",
  ns: "mydb.$cmd",
  // This is the UUID of the system.buckets collection
  ui: UUID("2aaf70a6-42ef-4933-8ef7-72e2a31c6093"),
  o: {
    upgradeDowngradeViewlessTimeseries: "myts",
    isUpgrade: true
  },
  // ...other common oplog fields
}

*DOWNGRADE FROM 9.0 TO 8.x*

{
  op: "c",
  ns: "mydb.$cmd",
  // This is the UUID of the viewless timeseries collection
  ui: UUID("2aaf70a6-42ef-4933-8ef7-72e2a31c6093"),
  o: {
    upgradeDowngradeViewlessTimeseries: "myts",
    isUpgrade: false
  },
  // ...other common oplog fields
}

Description of Linked Ticket

Building upon SERVER-114830, implement basic upgrade/downgrade support between viewful and viewless timeseries at the replica set level.

• Replication of upgrade/downgrade in the oplog via a custom 'c' entry - this allows secondaries to apply to upgrade/downgrade atomically.
• Integration into setFCV.

The goal of this ticket is to provide a targeted multiversion test that can upgrade/downgrade a timeseries collection.

Further test enablement will be done by follow up tickets because many operations (e.g. drop, timeseries read & write path, jstests themselves) do not expect the timeseries format to change mid-operation and will need to be fixed.