Fix exemplar support

[fcollonval]

Describe your environment

OS: Debian 12
Python version: Any
Package version: 0.49b0 and later

What happened?

Since 1.28.0, the opentelemetry Python SDK supports exemplars. However some instrumentation libraries does not add exemplars because the measure are added/recorded outside of the active span.

For example, in the asgi instrumentation, all measures should be taken within the use_span context rather than outside:

opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py

Lines 707 to 709 in ae13079

	self.active_requests_counter.add(1, active_requests_count_attrs)
	try:
	with trace.use_span(span, end_on_exit=False) as current_span:

Steps to Reproduce

You can test the repository https://github.com/webscit/opentelemetry-demo-python with OTEL_AUTO_INSTRUMENTATION_LEVEL = 1, it uses fastapi instrumentation (but the flaw is actually in the asgi instrumentation).

Expected Result

Exemplars are recorded for all Python instrumentations

Actual Result

Some instrumentations are missing exemplars

Additional context

I don't know if all packages or only a subset of them are concerned by this. I have at least identified 3 cases:

Package	Line to blame
asgi	opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py Line 709 in ae13079 with trace.use_span(span, end_on_exit=False) as current_span:
wsgi	opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-wsgi/src/opentelemetry/instrumentation/wsgi/__init__.py Line 664 in ae13079 with trace.use_span(span):
tornado	opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-tornado/src/opentelemetry/instrumentation/tornado/__init__.py Line 388 in ae13079 ctx = _start_span(tracer, handler)

Would you like to implement a fix?

None