Astro: Add support for Server Islands

[s1gr1d]

Description

Astro added Server Islands in version 4.12 (blog post here, RFC here). We can add some instrumentation around that.

Server Islands are similar like Client Islands, but should offer not only performance, but also personalization possibilities (meaning rendering not static, but dynamic HTML). First, placeholder content is loaded and second, this content is replaced with the HTML result of the deferred render. This is similar to Suspense in React but the difference is that the deferred island is a separate HTTP request.

What instrumentation could look like:

• Tracing Spans: Connected traces for Server Islands (server to client)
• Errors: should already be captured, but at least adding new tests

[matthewp]

Hello, let me know if there's anything I can help with this effort.