About support multi threads and auto finalize for skywalking-rust.

[jmjoy]

Refer to apache/skywalking-rust#20 (comment), I think the new api design is (fake code):

use skywalking_rust::context::trace_context::TracingContext;
use skywalking_rust::reporter::grpc::Reporter;
use tokio;

static TRACING_MANAGER: Lazy<TracingManager> = Lazy::new(|| TracingManager::default("svc", "ins"));

async fn handle_request(reporter: ContextReporter) {
    let mut ctx = TRACING_MANAGER.create_context();

    {
        // Generate an Entry Span when a request
        // is received. An Entry Span is generated only once per context.
        let span = ctx.create_entry_span("operation1").unwrap();

        // Something...

        {
            // Generates an Exit Span when executing an RPC.
            let span2 = ctx.create_exit_span("operation2").unwrap();
            
            // Something...

            // Drop span2 and finalize to ctx.
        }

        // Drop span and finalize to ctx.
    }

    // Create coroutine to handle async task.
    spawn(async move {
        let mut ctx2 = TRACING_MANAGER.create_context();

        {
            let span3 = ctx2.create_entry_span("operation3").unwrap();

            // Something...

            // Drop span2 and finalize to ctx2.
        }

        // Drop ctx2 and send to TRACING_MANAGER.
    });

    // Drop ctx and send to TRACING_MANAGER.
}

#[tokio::main]
async fn main() {
    let tx = Reporter::start("http://0.0.0.0:11800").await;

    // Start server...

    // Block to report.
    loop {
        let context = TRACING_MANAGER.receive_context();
        reporter.send(context).await;
    }
}

Add the TracingManager which should be singleton in the application, to manage the contexts, and use notify mechanism for reporting.

[wu-sheng]

Could you submit an issue(feature type) directly? As this is an official APIs discussion, it is better we keep them official and tracked by PMC team.

[jmjoy]

Done.

[wu-sheng]

For ppl reading this context, see #9297