[EPIC] Span V2 Support in Relay

[Dav1dde]

Background

In preparation for Span Streaming and the EAP Performance rework, there will be a new simplified span schema (let's call it SpanV2 for now).

The new span schema can be found here (TODO link to develop docs once its published).

Envelopes containing these standalone spans can contain spans from multiple segments and multiple different traces, there may be no connection between them. Relay needs to process them independently.

Requirements

Relay will need to:

• Enforce new standalone span limits
• Support SpanV2 schema
• Map from SpanV2 to Span
• Make the current standalone span processing work for those spans
• Make sure dynamic sampling works (the trace context is now unique for each span item in the header)

Design

On the protocol level we need a new SpanV2 item. The span item now has the DSC in its header, no longer the envelope.

Envelope Limits:

• Maximum 1000 spans
• Maximum 10 MiB envelope

SpanV2 will be (temporarily) mapped to the current span format and produced as such to Kafka.