Allow defining the "Active Span"

Problem Statement

Integrations such as SentryHttpMesageHandler rely on

_hub.GetSpan()?.StartChild

to create a span. This on the Hub today is wired as:

        public ISpan? GetSpan() => Transaction?.GetLastActiveSpan() ?? Transaction;

This means I can’t set a specific span to get returned from GetSpan. When parallelizing work, the parent/child relationship becomes messy since there are multiple spans “active” and relying on the root transaction isn’t possible to find out which one it is.

Solution Brainstorm

When we’re propagating the span explicitly downstream. A user can push a scope and set that new span as the active one. So integrations can find the correct parent span.

For example _hub.SetActiveSpan(span)

[KonstantinLukaschenko]

Related to #1679.

[mattjohnsonpint]

Fixed in #2364