feat(event-schema): Copy root span data when converting from events

[jan-auer]

SDKs place span data for root spans into event.contexts.trace.data.
Therefore, top-level span data was missing for metric extraction and in
indexed spans. This PR copies trace data over to the root span when
events are converted to spans.

The two types used for trace data and span data are different (called
Data and SpanData, respectively). Each of the types is partially
typed out, where some fields overlap but have different types, and
otherwise most fields are different. In a follow-up, these two types will be
merged, but in the meanwhile we can convert between them via IntoValue
and FromValue.

The current approach carries two noteworthy caveats:

1. Roundtrip behavior is loosened, since the conversion function places
   certain event attributes into span.data. These are then cloned back
   into trace.data, where they did not reside in the original event.
2. IntoValue does not honor skip_serialization, so all typed fields
   are inserted into the other field as Annotated::empty(). This is
   a cosmetic issue in tests and leads to suboptimal performance, but
   these fields will be omitted properly in JSON serialization.

Closes getsentry/sentry#73656

[jan-auer]

@phacops or @jjbayer, I'd ask for input on how to proceed. I'm not sure on the reasoning behind making the two data types separate, nor whether roundtrip behavior is actually needed anywhere. We do have a test for this, but from what I can see in the code base we never actually require proper roundtrips.

[jjbayer]

I'm in favor of merging SpanData and trace::Data (i.e. merging the data types instead of converting). They seem to serve the same purpose and they both need to retain arbitrary data, except for type mismatches on typed out fields.

We can break up the roundtrip test. We convert transactions to spans and have a feature that converts standalone spans to transactions (which we will hopefully remove soon), but one and the same data item is never roundtripped in real life.

[jan-auer]

Thanks, @cleptric also confirmed that we can merge these types.

have a feature that converts standalone spans to transactions

Can you point me to that? For this feature, do we need to remove some attributes that are currently not cloned over, such as the sentry.* attributes?

[jjbayer]

Thanks, @cleptric also confirmed that we can merge these types.

Can you point me to that?

relay/relay-server/src/services/processor/span/processing.rs

Lines 115 to 116 in 173a8da

	if should_extract_transactions && !item.transaction_extracted() {
	if let Some(transaction) = convert_to_transaction(&annotated_span) {

For this feature, do we need to remove some attributes that are currently not cloned over, such as the sentry.* attributes?

No I think it's fine if we fully copy span.data intro contexts.trace.data for this feature. Next week I will look into removing this feature.