Clarify the docs better for how to use traceSampler() functions

[Dhrumil-Sentry]

Problem Statement

Got this feedback on discord and I felt our docs mention just use span.name but the product unfortunately does not have span.name yet - So we should atleast tell the user show to find this field for what they're trying to filter

Solution Brainstorm

Add hints on how to see the name/attributes of spans - and use span.desc for now?

[Lms24]

Transferring to docs, as this doesn't require SDK changes.

Add hints on how to see the name/attributes of spans

The code example in docs already shows that users can access attributes from the passed sampling context variable. Do you have a concrete idea what we should show here? I can add another condition that checks for a specific attribute for example.

and use span.desc for now

we can't because at this point (i.e. shortly before the SDK starts the span), the span only has a name, which users or our instrumentation will pass to startSpan({name: '...'}, () => {}). We should also keep it this way because this is the same API used in OpenTelemetry. Only later on, when the span ends and it is serialized, the name is converted to the span description. Not ideal for users, I agree, but long-term we'll move to name only as far as I understand.