feat(protocol): Add new frame fields for metric kit flamegraphs

[noahsmartin]

MetricKit provides app hang data on iOS which the cocoa SDK reports as an event. However, the stacktrace metric kit provides is really a "call stack tree" each node has a sample count and a parent. The best way to visualize it is a flamegraph, so this PR adds the relevant fields to relay allowing us to show the flamegraph on the issue detail page.

It doesn't look that useful yet because the frames are not symbolicated when I run it locally, but once this is merged I'll be able to run it on prod data with the frontend updates to see a symbolicated flamegraph:

Another way of doing this that I considered when chatting with @jan-auer was to include this in the frame data but I tried that and it didn't seem like the fields in data were preserved. This seems to work well though, and IMO is a bit cleaner than just throwing it in data.

WIP Frontend PR: getsentry/sentry#106412
WIP SDK PR: getsentry/sentry-cocoa#7185

[Dav1dde]

Would be good if you can also add a small changelog message.

[jjbayer]

I believe this should this go into FrameData: "Auxiliary information about the frame that is platform specific."

relay/relay-event-schema/src/protocol/stacktrace.rs

Lines 132 to 135 in 3a1981c

	/// Auxiliary information about the frame that is platform specific.
	#[metastructure(omit_from_schema)]
	#[metastructure(skip_serialization = "empty")]
	pub data: Annotated<FrameData>,

[jjbayer]

My previous comment might be wrong, the doc on FrameData itself mention that it should not be set by SDKs:

relay/relay-event-schema/src/protocol/stacktrace.rs

Lines 209 to 211 in 3a1981c

	/// Additional frame data information.
	///
	/// This value is set by the server and should not be set by the SDK.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}

[cursor]

Frame indices break after stacktrace trimming

Medium Severity

parent_index encodes positional relationships within stacktrace.frames, but existing normalization/trimming logic can remove frames from the middle of the list. After trimming, parent_index can point at the wrong frame or a removed one, breaking MetricKit flamegraph reconstruction and potentially causing downstream consumers to error.