[SDT-0001]: task-local instrument implementation

[kukushechkin]

Add withInstrument(_:_:) for task-local instrument scoping.

Motivation:

InstrumentationSystem.bootstrap(_:) installs an instrument once per process, so every withSpan/startSpan and every read of InstrumentationSystem.instrument resolves through that single instrument. That fits one tracer for the whole application, but it leaves no way to bind a different instrument for a region of work — most acutely in tests, where a second bootstrap crashes and parallel tests cannot each install their own in-memory tracer.

Modifications:

• Added withInstrument(_:_:) and an internal TaskLocalInstrument.
• InstrumentationSystem installs the task-local instrument on first use, and tracer discovery and propagation resolve through it via an internal _InstrumentContainer walk shared with MultiplexInstrument.
• Added tests covering scoping, replacement, MultiplexInstrument resolution, and the self-reference guard, and updated the TraceYourApplication and InstrumentYourLibrary guides.

Result:

swift-distributed-tracing provides opt-in withInstrument(_:_:) task-local instrument scoping.