Spawn tasks with current tracing subscriber

[mdup]

Before this PR, tracing logs work with the default subscriber (which is the usual case), but when one sets a custom tracing subscriber, it isn't passed across tokio::spawn boundaries.

In this PR I'm adding the method .with_current_subscriber() to server tasks. This allows the logging subscriber to be carried into the spawned tasks.

// Before: subscriber context lost
tokio::spawn(async { ... });

// After: subscriber context preserved  
tokio::spawn(async { ... }.with_current_subscriber());

My use case is capturing logging for integration tests, for which I need to set a custom tracing subscriber. I think this PR shouldn't change anything in the common case of a global subscriber.