Record the object store and server name of a backup in Backup.status.pluginMetadata #1140

Description

A Backup taken through the plugin doesn't record which ObjectStore and which serverName the backup was written to. When creating a recovery cluster, we have to find that out by other means and write it by hand in the externalClusters entry.
I'd like the plugin to add this information to the backup result metadata, so that it ends up in Backup.status.pluginMetadata.

This is related to #510 and cloudnative-pg/cloudnative-pg#7989 (recovery from a plugin Backup object), which I believe need this as a first step.

Our use case

Our clusters archive under a timestamped serverName (for example mydb-20261003T120000), set through the serverName plugin parameter, and a new one is generated for each new generation of a cluster.
The archive path has to be empty when a cluster starts archiving, and a fresh, dated serverName gives us that without deleting older archives.
Over time, the bucket holds one folder per generation.

To create a recovery cluster from a given backup, we need to know which of these folders holds it. The plugin's recovery default for serverName is the name of the new cluster (internal/cnpgi/operator/config/config.go), which never matches a timestamp, so we have to find the value by listing the bucket or
digging through old manifests.
The Backup object is the natural place to keep the link between a backup and its location, and it's where the in-tree integration kept it: Backup.status.serverName, destinationPath and endpointURL were all set. Since the in-tree integration is deprecated since 1.26, anyone migrating to the plugin loses that.

Environment

• CloudNativePG operator: main (the behaviour is the same on current releases)
• plugin-barman-cloud: v0.15.1

What a plugin backup records today

apiVersion: postgresql.cnpg.io/v1
kind: Backup
spec:
  method: plugin
  pluginConfiguration:
    name: barman-cloud.cloudnative-pg.io
status:
  backupId: ...
  pluginMetadata:
    clusterUID: ...
    displayName: ...
    name: ...
    pluginName: ...
    timeline: "1"
    version: ...

Nothing in it says where the backup went. newBackupResultMetadata (internal/cnpgi/instance/types.go) is where these keys are built.

Proposal

Add the configuration the backup was taken with to the same map:

This only adds keys and doesn't change any existing behaviour. destinationPath and endpointURL aren't strictly needed to locate the data, since they can be read from the ObjectStore, but they let a later restore detect that the ObjectStore has been changed to point somewhere else since the backup was taken.

Shipping this early matters even if nothing consumes it yet: backups taken before it lands will never carry the information.

Alternatives considered

• Dedicated fields in Backup.status (serverName, destinationPath): they would need new fields in the CNPG-I BackupResult message for concepts that are specific to Barman, so pluginMetadata looks like the right place.
• Keep the status quo and document that serverName must always be set in the recovery source.
  This is what we do today, but with timestamped names the value has to be found outside the cluster.
• Change the default recovery serverName: no default can guess a timestamp, and changing it would break users who rely on the current one.

I used an AI assistant (Claude) to help draft this issue and to check the references against the plugin and operator sources, as the AI policy asks for disclosure.