docs: drop telemetry: mention COSAgentProvider explicitly

*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](https://github.com/canonical/grafana-agent-operator/blob/8cbc7cf331d6e1e0c543d1d763892c56ba507670/lib/charms/grafana_agent/v0/cos_agent.py#L72) to pass to COSAgentProvider
- In your own generated [scrape_config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config) include a [metric_relabel_configs](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#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/*