Temporarily Default to XRay Remote Sampler is Sampler is not Specified

[jj22ee]

Issue #, if available:
Use X-Ray default sampler as default sampler if user has not specified one
Requires #55 to be merged first before this PR can work.

Description of changes:
Use X-Ray default sampler as default sampler if user has not specified one

Testing:
Assume #55 is merged

1. Enabled span_metrics_processor and added debug statement to print out Sampled status of a span in on_start()

# aws_span_metrics_processor.py
if span.get_span_context().trace_flags.sampled:
            print("sampled")

2. Replaced resource detectors and manually set resource with service.name=test-service-name and build ADOT SDK
3. Setup OTel collector with XRay proxy for sampling and AWS credentials
4. In AWS account, create sampling rule to match service_name=test-service-name
5. Setup sample app in sample-applications/simple-client-server/server_automatic_s3client.py
6. Update sample-applications/simple-client-server/client.py to call server/sample-app a variable number of times to verify sampling rule is applied
7. Repeat 6 after changing sampling rule rate/reservoir

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[jj22ee]

Do not merge until #55 is merged.

[thpierce]

Will rely on Prashant for the full review, just one comment from my quick skim.

[srprash]

Minor comments on the UTs. Otherwise LGTM!