fix: Hard-coded metrics port on the standalone chart

[as51340]

Uses predefined configuration variable instead of the hard-coded 9091 port.

[as51340]

Tracking

• [Link to Epic/Issue]

Standard development

• Update / add chart templates, values, and NOTES.txt as needed
• Run helm lint and helm template locally against the changed chart(s)
• Verify the chart installs and upgrades cleanly on a test cluster

Labels checklist

• Add a docs label (exactly one): docs-changelog-only, docs-needed, or docs-not-needed
• Add at least one component label: memgraph, memgraph-ha, memgraph-lab, or infrastructure
• Add at least one type label: bug, feature, or infrastructure
• Assign the PR to a milestone
  • If not known, set for a later milestone

Documentation checklist

• Write a release note, including added/changed clauses
  • Mg exporter in the standalone chart will now use predefined variable instead of the hard-coded 9091 port. #266
  • What has changed? What does it mean for a user? What should a user do with it? [#{{PR_number}}]({{link to the PR}})
• [ Documentation PR link memgraph/documentation#XXXX ]
  • Is back linked to this development PR