Return usable/recording spans from the agent context

[dmathieu]

This is a fix for #1476.

[wzy9607]

This will likely result in nil pointer panic if some Otel instrumentation attempts to use the provider from trace.SpanFromContext(ctx).TracerProvider().

I searched https://github.com/open-telemetry/opentelemetry-go-contrib, and found that one instrumentation indeed uses it. See
https://github.com/open-telemetry/opentelemetry-go-contrib/blob/df37a9d942c240f6c7ebdcee1900368728dd3adb/instrumentation/net/http/httptrace/otelhttptrace/clienttrace.go#L163-L176
https://github.com/open-telemetry/opentelemetry-go-contrib/blob/df37a9d942c240f6c7ebdcee1900368728dd3adb/instrumentation/net/http/otelhttp/transport.go#L96-L102

[dmathieu]

Good point. I've added the global tracer provider, if it's an apmotel one.
If it isn't one, we can't retrieve it from the global context.