Document technique for associating queries with consuming applications via request-id header

[cjcenizal]

When multiple applications are querying the same corpus, it becomes difficult to discern which applications sent which query. To address this, we expect developers to append (x-request-id headers](https://http.dev/x-request-id#takeaway) to their requests to enable them to tell which application sent which query.

We should document this technique so developers can learn about it.

[tallatshafaat]

x-request-id can be used for basic stuff, but any advance usage won't be possible with it. E.g., users might want to add multiple tags to a query, not just one.

Also note that today, the request-id is not exposed directly in the query history. E.g., you cannot filter by it. So even if you added the username as x-request-id, you cannot get all queries for the username.