fix(telegraf): use subPath for telegraf.conf to support additional config mounts

[sotoiwa]

Fix: #721

Summary

This PR updates the volumeMounts for telegraf.conf to use subPath, allowing users to mount additional configuration files (e.g., for inputs and outputs) under /etc/telegraf/telegraf.d.

Problem

Currently, the Helm chart mounts the full ConfigMap at /etc/telegraf:

    volumeMounts:
      - name: config
        mountPath: /etc/telegraf

This prevents mounting any other files under /etc/telegraf, such as custom .conf files.

Solution

By changing the mount to:

    volumeMounts:
      - name: config
        mountPath: /etc/telegraf/telegraf.conf
        subPath: telegraf.conf
        readOnly: true

we allow users to mount additional configuration files (e.g., /etc/telegraf/telegraf.d/*.conf) in parallel without conflict.

Benefits

• Supports mounting supplemental configs via separate ConfigMaps
• Promotes flexibility and extensibility for more complex Telegraf setups

Notes

• readOnly: true is explicitly set for safety when using subPath

[skartikey]

@sotoiwa
Issues/suggestions:

1. Missing version bump - Chart.yaml still shows version 1.8.65. Per the contributing guidelines, this needs to be bumped for release
2. Inconsistency with telegraf-ds - The telegraf-ds chart (line 65 of daemonset.yaml) still uses:

• name: config
  mountPath: /etc/telegraf

3. This creates behavioural divergence between similar charts
4. Potential breaking change - Users who extended the ConfigMap via kustomize overlays or Helm post-rendering to include additional files would lose access to them. This is an edge case but worth noting in release notes
5. Documentation gap - The chart README should be updated to document:
   - This mount behaviour change
   - How to mount additional configs in /etc/telegraf/telegraf.d/