Switch grafana-agent to opentelemetry-collector

[Pjack]

Please switch grafana-agent to opentelemetry-collector
https://github.com/search?q=repo%3Acanonical%2Fopenstack-exporter-operator%20grafana-agent&type=code

Reference: https://discourse.canonical.com/t/charm-engineering-lt-notes-7-jan-2026/6817

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/SOLENG-1778.

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