add User-agent OS attributes

[rogercoll]

Fixes #1433

Changes

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• CONTRIBUTING.md guidelines followed.
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• schema-next.yaml updated with changes to existing conventions.

[rogercoll]

The PR is mostly a copy and paste from the OS attributes current definition to the user_agent's attributes, is there a way to reference them but applying the "user_agent" prefix?

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[lmolkova]

Could you please describe the use-case for these attributes? is there any instrumentation or semantic convention that's supposed to populate it?

We usually add attributes along with the convention (span/metric/resource/event) that uses them - this usually results in more specific attribute definition and more clarity for those who want to populate the attribute on other conventions.

So, could we define such convention and/or document the use-case in some other way?

[rogercoll]

Could you please describe the use-case for these attributes?

For example, to understand which are the source OSes that are making requests to your system. This could be used from a Nginx Ingress Controller log, which records all requests UserAgent's.

is there any instrumentation or semantic convention that's supposed to populate it?

There is an ongoing PR in the collector: open-telemetry/opentelemetry-collector-contrib#35458

We usually add attributes along with the convention (span/metric/resource/event) that uses them - this usually results in more specific attribute definition and more clarity for those who want to populate the attribute on other conventions.

From the collector point of view, these attributes are agnostic to its location, they will be extracted from any defined context (span/metric/resource/event). I would like to use it to populate a Nginx Ingress Controller event, but it is not defined in Semconv yet (#982). Would it make sense to add a Nginx log example and explain how these attributes could be extracted?