Telemetry Processor: What telemetry data can be combined

[philipphofmann]

Define more clearly either in the telemetry processor develop docs or somewhere else in the telemetry docs and link to this info from the telemetry processor, to clearly inform SDK developes which types of telemtery data SDKs can combine in envelopes, because this impacts the design of the telemetry processor as it needs to know what it can put into an envelope. For example, an envelope with spans can only contain spans from a single traceID, while this isn't true for logs and metrics.

[linear]

DOCS-2402

[philipphofmann]

The general rule is we must not combine different types of telemetry data together into the same envelope because the rate limit enforcement on Relay then gets too expensive, but there are a few exceptions, such as session, client reports, attachments, etc.