feat: add stage.useragent

[pi12138]

PR Description

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• [ ] CHANGELOG.md updated
• [x] Documentation added
• [x] Tests updated
• [ ] Config converters updated

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:x: pi12138

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[clayton-cornell]

This will need documentation. The loki.process stage docs are here: https://grafana.com/docs/alloy/latest/reference/components/loki/loki.process/ and in the repo at /docs/sources/reference/components/loki/loki.process.md

[pi12138]

This will need documentation. The loki.process stage docs are here: https://grafana.com/docs/alloy/latest/reference/components/loki/loki.process/ and in the repo at /docs/sources/reference/components/loki/loki.process.md

Documentation updated

[clayton-cornell]

@pi12138 Docs look OK, this has to go through code review with the @grafana/grafana-alloy-maintainers