Update o11y dev guides with latest mocks

What does this MR do and why?

I was updating the local mock patch and ended up removing the snippet by mistake 🙈

Here's the latest version.

I've cleaned up the mock by replacing it with axios-mock-adapter and moving almost all of the patched code outside the client, so that it should not keep breaking every time we touch the client

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

NA

How to set up and validate locally

Verify mocks still work:

git apply < <(curl --silent "https://gitlab.com/gitlab-org/opstrace/opstrace/-/snippets/3747939/raw/main/mock.patch")