feat(tracing without performance): set span on scope

[greywolve]

This is part of a series of PRs for the "tracing without performance" feature.

For the case where performance is enabled we need to be able to get the tracing context from the active transaction and use that instead of the propagation context. So we need to way to access the span.

This PR adds support for setting a span on a scope. We can then access the span and get the current tracing context to pass through.

[codecov]

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