feat(trace): trace requests via langsmith

What does this merge request do and why?

This MR introduces a middleware for distributed tracing. We can set various context for the purpose, and as a starter we start supporting LangSmith's distributed tracing.

GitLab-Rails counter-part: Send LangSmith Run ID to AI Gateway (gitlab-org/gitlab!161886 - merged)

Related to Add support for LangSmith tracing in AI Gateway (#559 - closed)

How to set up and validate locally

1. Checkout gitlab-org/gitlab!161886 (merged) and run GDK.

Here is an example of trace:

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.