fix(helm): improve documentation and grafana dashboard (#2992)

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---

* **docs**: enabling of metrics in there config is needed
* **dashboard**: since the imported dashboard was created:
  * many metrics was dropped by dendrite (mainly #2967)
  * grafana has new version of diagrams ...

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Co-authored-by: Till <2353100+S7evinK@users.noreply.github.com>

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