Enhance OTel spans with an identifier for logged-in users

[mikerkelly]

Why are we doing this?

Honeycomb allows us to view OTel data exposed by the site. This allows us to monitor performance. This will be enhanced if we can view traces and spans by who is making them. This may help us identify users facing particular issues, for example.

How will we know when it's done?

• OpenCodelists Honeycomb queries can use logged in user as a search term.
• We have better developer documentation of how we OTel, Honeycomb, and how to develop against them.

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