Add `getTracingMetaTags` helper function to Node SDK

[Lms24]

Problem Statement

Sentry's browser SDKs pick up <meta name="sentry-trace" .../> and <meta name="baggage" /> tags in a users' HTML when starting the initial pageload transaction. This is how SSR transactions can be connected to their frontend pageload transactions. We use this mechanism internally in our Full-Stack framework SDKs (NextJS, Remix, SvelteKit) but also users can add this to their own SSR setup.

Right now, this feature is neither well documented nor easy to use on the server side.

One use case for the Node SDK: An express app using our Node SDK that renders HTML pages.

### Tasks
- [x] Add a helper function that renders the Meta tags to a string that users can simply call and then include in their HTML.
- [ ] https://github.com/getsentry/sentry-docs/issues/7671

[swantzter]

Hi, This seems like it could be a relevant update in time for v8 with the multiple deprecations and removals of API's used to get those headers. I'm trying to implement the generation of these headers in our in-house SSR framework right now and am running into the issue that, well, all the methods described on https://docs.sentry.io/platforms/node/distributed-tracing/custom-instrumentation/#inject-tracing-information-to-outgoing-requests to get the sentry-trace and baggage headers seems to be deprecated.

I found #10031 which adds a new way to get the sentry-trace header/meta value, but I've yet to find the replacement for dynamicSamplingContextToSentryBaggageHeader which (no longer?) seems importable from @sentry/node nor @sentry/core?

[swantzter]

Oh! I see there's work on this in https://github.com/getsentry/sentry-javascript/blob/3d82cdb8a4c5d1642976332b6c522498c99901bd/packages/astro/src/server/meta.ts already, I'll copy that to my code for now.

A feedback for when that moves to core, it'd be nice to be able to get the values as unwrapped strings, I would like to be able to use it with something like unhead / useHead

[Lms24]

Hey @swantzter thx for sharing your thoughts!. First of all, apologies for the outdated docs around obtaining the meta tag values - these are no longer valid in version 8 of the SDK and in v7, most of the APIs are deprecated. We're unfortunately lagging a bit behind with docs at the moment but we have a huge list of doc updates we'll work through soon (#11064).

The snippet you found in the Astro SDK is spot on! As the JSDoc suggests, we'll move this implementation to core sometime soon (TM).

A feedback for when that moves to core, it'd be nice to be able to get the values as unwrapped strings, I would like to be able to use it with something like unhead / useHead

That's valuable feedback indeed, thanks! I think Remix exposes similar functionality there's already two use cases to obtain the tags as objects rather than stringified html. However, there's also demand for the fully stringified version we currently use in the Astro package (and a couple of other packages).

I think we ideally expose:

• One function that just obtains all tracing-relevant data for a given span, scope and client. This returns the contents of the sentry-trace and baggage <meta> tags or Http headers. The values are the same in both cases and I think we have enough internal cases where this would also be super helpful in addition to exporting it outside.
  Mental model: One function call to obtain all "raw" tracing data - perfect for your described use case
• One function that stringifies the returned values of the function above to <meta> tags. We need this mechanism at least twice in our own code so it probably also makes sense to expose this. Mental model: One function call to inject pre-built meta tags.

[swantzter]

@Lms24 absolutely, exposing two functions or one function with two different possible return types depending on options was what I had in mind as well, that sounds promising :)