Add Inferred spans extension

[JonasKunz]

Adds the inferred spans feature from the previous elastic-apm-agent as a standalone OTel-extension.
Closes #23. Autoconfiguration and a README.md as documentation will be added with #29 .

The initial commit of this PR is an unchanged copy of all the files from the original profiling plugin, reformatted with spotless. This should allow you to ignore this commit when reviewing to focus on the more relevant remainign diff.

Below is a sample application on how this extension can be used and tested.
I've also used this example application to manually verify that the backup diagnostic files feature works and the diagnostic files can be replayed.

Sample app

public class InferredSpansTestApp {

    public static void main(String[] args) {
        InferredSpansProcessor profiler = InferredSpansProcessor.builder()
                .samplingInterval(Duration.ofMillis(10))
                //.backupDiagnosticFiles(true)
                .build();
        SdkTracerProvider tracerProvider = SdkTracerProvider.builder()
                .setResource(Resource.builder()
                        .put(ResourceAttributes.SERVICE_NAME, "Inferred Spans Otel Test")
                        .build())
                .addSpanProcessor(profiler)
                .addSpanProcessor(BatchSpanProcessor.builder(OtlpGrpcSpanExporter.builder()
                                .setEndpoint("https://XXXXXX.apm.europe-west3.gcp.cloud.es.io:443")
                                .addHeader("Authorization", "Bearer XXXXX")
                                .build()
                        )
                        .build())
                .build();
        profiler.setTracerProvider(tracerProvider);

        try (OpenTelemetrySdk sdk = OpenTelemetrySdk.builder()
                .setTracerProvider(tracerProvider)
                .build()) {
            while (true) {
                doTracing(sdk.getTracer("my-manual-tracer"));
            }
        }
    }

    private static void doTracing(Tracer tracer) {
        Span span = tracer.spanBuilder("my-root").startSpan();
        try (var scope = span.makeCurrent()) {
            doSleep(100);
            inferMeParent(tracer);
        } finally {
            span.end();
        }
    }

    private static void inferMeParent(Tracer tracer) {
        inferMe(tracer);
    }

    private static void inferMe(Tracer tracer) {
        child1(tracer);
        inferMeToo();
        child2(tracer);
    }

    private static void child1(Tracer tracer) {
        Span span1 = tracer.spanBuilder("child-1").startSpan();
        try (var scope = span1.makeCurrent()) {
            doSleep(200);
        } finally {
            span1.end();
        }
    }

    private static void child2(Tracer tracer) {
        Span span2 = tracer.spanBuilder("child-2").startSpan();
        try (var scope = span2.makeCurrent()) {
            doSleep(400);
        } finally {
            span2.end();
        }
    }


    private static void inferMeToo() {
        doSleep(300);
    }

    private static void doSleep(long millis) {
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

[JonasKunz]

This file has also been copied unchaged.

[JonasKunz]

This method was the only change/addition in the pooling package, all other classes have been copied unchanged.

[JonasKunz]

This class also has been copied unchanged from the elastic apm agent

[JonasKunz]

Related apm-data PR: apm-data/pull/185

[SylvainJuge]

[minor] why not do the change right now ?

[JonasKunz]

ExecutorUtils is not yet visible from here, because it is in the custom subproject. We'll need to move ExecutorUtils to a common project first.