Align spans with SRA and add tests (#4052)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Update our tracing spans to better align with the SRA.

## Description
<!--- Describe your changes in detail -->
* Add top level `{ServiceName}.{OperationName}` span with various SRA
mandated attributes
* Make all spans debug level (was previously mix of debug and info)
* Add some useful fields to spans (attempt_number, etc.)
* Add tests for span behavior (checking nesting structure, number of
times span appears, etc.)

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Added tests for span emission logic in the `integration-tests/metrics`
test suite

## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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