Enable struct Spans with reference semantics; optimal NoopSpan

[ktoso]

Enable struct Spans with reference semantics; optimal NoopSpan - as per @fabianfett's request recently

Motivation:

We want a noop span to be NOT ref counted.

Modifications:

Allow spans to be value types through they must have reference semantics

Result:

Efficient noop spans

---

Also, Span as the protocol name again

[ktoso]

magic.

[ktoso]

"whoa whoa whoa" this saved the day.

[fabianfett]

🥳

[stevapple]

I believe we need some "Swift Protocol Design Guidelines" that’s going to save us from naming🤣

[ktoso]

Yeah the protocol naming is rough when one uses any as well as associated types AND concrete ones would like to share the same name... but this will work okey i think

[ktoso]

For a dumb benchmark like this:

@_dynamicReplacement(for: registerBenchmarks) // Register benchmarks
func benchmarks() {
    Benchmark("noop spans: 1") { _ in
        _ = Tracer.withSpan("hello-1") { span in
            _ = test(span: span)
        }
    }

    Benchmark("noop spans: 3") { _ in
        _ = Tracer.withSpan("hello-1") { span in
            _ = Tracer.withSpan("hello-1-2") { span in
                test(span: span)
            }
            _ = Tracer.withSpan("hello-1-2") { span in
                test(span: span)
            }
            test(span: span)
        }
    }

    Benchmark("noop spans: 10") { _ in
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = Tracer.withSpan("hello-1") { span in
            test(span: span)
        }
    }

    let noop: NoOpTracer = NoOpTracer()
    Benchmark("concrete spans: 10") { _ in
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
        _ = noop.withSpan("hello-1") { span in
            test(span: span)
        }
    }
}

@inline(never)
func test(span: any SpanProtocol) -> Int {
    12
}

Would be actually fun to hook the refcount funcs as well, so we could see reduction in refcounts explicitly.

[ktoso]

Talked with @fabianfett, I believe we're happy here 👍