tracing's span could be propagated through RPC to Sqlite worker

[Andrepuel]

On Mysql driver, when a span is entered, given that the appropriate care is taken regarding asynchronous functions, then all the events will be sent using the span that was entered.

Sqlite driver, however, will always emit events on the root span. This seems to be related to the fact that the actual queries are executed in another thread in a worker. The worker does not know which span was entered when the original caller sent the request.

My proposal is that the current span should be sent alongside the work request so that the worker may enter the same span that was entered on the caller site.

I think the effort could be concentrated in the Command enum: https://github.com/launchbadge/sqlx/blob/v0.7.1/sqlx-sqlite/src/connection/worker.rs#L42 .