docs: drop telemetry: mention COSAgentProvider explicitly

[sed-i]

Please describe the question or issue you're facing with "Selectively drop telemetry - Observability documentation".

For completeness, we should mention COSAgentProvider in the docs, something along the lines of:

• Use cos-agent library's COSAgentProvider
  • In your charm's code, generate scrape_config to pass to COSAgentProvider
    • In your own generated scrape_config include a metric_relabel_configs section with e.g. drop directive.

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Reported from: https://documentation.ubuntu.com/observability/latest/how-to/selectively-drop-telemetry/