Adding AwsUnsampledOnlySpanProcessor to export batches of unsampled spans

[srprash]

Description of changes

Following the approach as ADOT Python: aws-observability/aws-otel-python-instrumentation#247. Note that the span attribute key was later changed to aws.trace.flag.sampled.

Introducing AwsUnsampledOnlySpanProcessor that can be appended to a TracerProvider. This span processor wraps a BatchSpanProcessor for delegating all the operations after processing. The AwsUnsampledOnlySpanProcessor does 2 things:

• onStart - Sets the aws.trace.flag.sampled span attribute on each span.
• onEnd - If the span is unsampled, only then it is delegated to the BSP, otherwise it is dropped.

Testing

Setup

@Test
  public void testExportingUnsampledSpan() {
    SpanProcessor unsampledSpanProcessor = AwsUnsampledOnlySpanProcessor.builder().build();

    SdkTracerProvider tp =
        SdkTracerProvider.builder()
            .addSpanProcessor(unsampledSpanProcessor)
            .setSampler(AlwaysRecordSampler.create(Sampler.alwaysOff())) // change to alwaysOn to sample
            .build();

    Tracer tracer = OpenTelemetrySdk.builder().setTracerProvider(tp).build().getTracer("hello");

    Span span = tracer.spanBuilder("MySpan_unsampled_006").startSpan();
    span.setAttribute("foo", "bar");
    span.end();

    unsampledSpanProcessor.forceFlush();

    // add 5 seconds delay
    try {
      Thread.sleep(5000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }

Observations:

• Unsampled span is exported in a batch.
• Switching to alwaysOn sampler, the sampled span is not exported at all.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Codecov Report

Attention: Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.38%. Comparing base (09e6487) to head (d236e90).
Report is 368 commits behind head on main.

Files with missing lines	Patch %	Lines
...agent/providers/AwsUnsampledOnlySpanProcessor.java	68.42%	6 Missing ⚠️

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Additional details and impacted files

@@              Coverage Diff              @@
##               main     #948       +/-   ##
=============================================
- Coverage     85.71%   71.38%   -14.33%     
- Complexity       19      303      +284     
=============================================
  Files             3       26       +23     
  Lines            49     1094     +1045     
  Branches          5      145      +140     
=============================================
+ Hits             42      781      +739     
- Misses            3      267      +264     
- Partials          4       46       +42     

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