client: add support for iterating client metadata keys

[axw]

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Describe the issue you're reporting

The https://pkg.go.dev/go.opentelemetry.io/collector/client#Metadata type provides no means of iterating over the metadata keys -- you can only call Get with a known key. Being able to iterate the keys would be necessary to implement open-telemetry/opentelemetry-collector-contrib#39157, i.e. in order to obtain request.metadata, the full map of client metadata.

I propose adding a func (Metadata) Keys() iter.Seq[string] method that iterates over the metadata keys. See #12805.