Implement Unit Test for AWS Metric Attributes Span Exporter

[XinRanZhAWS]

Description of changes:
Implement Unit Test for AWS Metric Attributes Span Exporter

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[zzhlogin]

Please go over the file:

1. add type hint for all variables when it is first initialized.
2. add all comments the same as Java tests.
3. There should be no module level function, please move them into the class and mark as internal use starting with _. Same applies for variables.

[zzhlogin]

Please add one more test to address this :

aws-otel-python-instrumentation/aws-opentelemetry-distro/src/amazon/opentelemetry/distro/aws_metric_attributes_span_exporter.py

Line 105 in 3c822a8

# TODO: AwsMetricAttributesSpanExporter depends on internal ReadableSpan method _attributes.

[zzhlogin]

I only add comments for partial of the files. Please go over the file carefully, and add the type hint for all variable initialization that is missing .