Fix goroutine leak in local.file

[wildum]

PR Description

Call UpdateArgs instead of Update in the New() function to avoid starting goroutines before Run() is called

Which issue(s) this PR fixes

Fixes grafana/alloy#247

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated

[rfratto]

Thanks for taking a crack at this! I find this a little hard to reason about, as now I have to track down to make sure no fields set in Update are needed at construction time, and that they're all set once Run is fully in motion.

Instead of creating two updates, can we try to prevent the goroutine from being started until Run is called?

[ptodev]

I think Run() could be called without calling Update(). In that case, is the fact that we didn't call c.configureDetector() prior to Run() a problem?

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