## What this PR does
PR #2075 introduced `_cluster.cluster-domain` references in
monitoring-agents `values.yaml` for FQDN resolution in tenant clusters.
This broke the fluent-bit subchart because `_cluster` values are not
accessible from the Helm subchart context — only `global` values are
shared with subcharts.
This PR replaces `_cluster` references with a new `global.clusterDomain`
variable:
- Empty by default (management cluster uses short DNS names like
`service.namespace.svc`)
- Set to the management cluster domain (e.g. `cozy.local`) for tenant
clusters, enabling FQDN resolution for cross-cluster service discovery
Fixes#2084
### Release note
```release-note
[system] Fix monitoring-agents installation failure caused by inaccessible _cluster values in fluent-bit subchart context. Introduce global.clusterDomain for proper FQDN resolution in tenant workload clusters.
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Chores**
* Monitoring agent configuration updated to support configurable cluster
domain names for greater flexibility.
* Remote write and log-forwarding endpoints adjusted to align with
cluster domain handling, improving compatibility when deploying across
different cluster DNS setups.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Andrei Kvapil
d8f5083c6d