Add fastapi arguments attributes directly on the root otel span

[alexmojaki]

Having a separate "FastAPI arguments" span is a pain. It costs extra money and requires writing a join in SQL to combine info about the root span (particularly the duration) with the argument values. I want to get rid of it.

As a first step, this PR takes the same attributes that are set on the fastapi args span and sets them on the root span created by OTEL. This allows people to use the root span in queries and avoid joining in SQL, and should ease the transition to removing the fastapi args span.

In a followup change I will remove the fastapi args span. We might want to treat this as a breaking change and release version 2.0. We might also want to add a parameter (off by default) that keeps the span, although the only reasons I can think of to keep it are quite weak:

1. Backwards compatibility in dashboard/alert queries
2. Grouping together spans that come from resolving arguments (e.g. db queries)
3. Seeing how long it takes to resolve arguments

[cloudflare-workers-and-pages]

Deploying logfire-docs with    Cloudflare Pages

Latest commit:	cecc093
Status:	✅  Deploy successful!
Preview URL:	https://72333100.logfire-docs.pages.dev
Branch Preview URL:	https://alex-fastapi-args-root-span.logfire-docs.pages.dev

View logs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6d7bb92) to head (cecc093).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #509   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          132       132           
  Lines        10056     10082   +26     
  Branches      1360      1364    +4     
=========================================
+ Hits         10056     10082   +26     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Kludex]

Nice!

[alexmojaki]

This option was a bad idea on my part, clutters the docs and API, and doesn't really have a purpose any more now that OTEL kwargs are passed through.

[alexmojaki]

After chatting with @dmontagu I think it's likely that we won't remove the fastapi args span soon, but this change should be useful anyway.

[Kludex]

We might also want to add a parameter (off by default) that keeps the span

I don't think this is necessary...

[JacobHayes]

Ah, this might also fix #533 if it also removes the http.method and http.route attributes from the FastAPI Arguments span.

---

Separately, I do find it somewhat useful to track the db queries that come from parsing arguments and resolving Depends (especially since optimizing those would often help multiple endpoints), but would be fine if that was dropped.

[alexmojaki]

Separately, I do find it somewhat useful to track the db queries that come from parsing arguments and resolving Depends

But @JacobHayes if you've instrumented db queries then you'll still see spans for those queries even if we drop the fastapi args span, they just won't all be grouped together. Otherwise you'll see fastapi args taking a long time to resolve but you won't know why.