Add support for managing OpenTelemetry attributes

[stevehipwell]

Is your feature request related to a problem? Please describe.

I'd like to be able to export logs to Grafana via the OpenTelemetry format and this requires the ability to set OpenTelemetry attributes.

Describe the solution you'd like

I'd like the ability to both modify attributes as a filter and as part of the OpenTelemetry output.

Describe alternatives you've considered

I can use the Loki output but this isn't a documented log ingestion pattern in Grafana Cloud so support might be removed at any point.

Additional context

[github-actions]

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[stevehipwell]

@patrick-stephens this is the issue for grafana/loki#4648 (comment).