remove redundant span attributes and improve log output format (#4075)

## Description
https://github.com/smithy-lang/smithy-rs/pull/4052 aligned our spans
closer to SRA but the output format resulted in `rpc.service` and
`rpc.method` on both the new generated operation span and the internal
orchestrator `invoke` span. This resulted in pretty long and clutter log
output. This PR cleans up the log output a bit to have only the top
level operation span include these attributes. This also fixes a subtle
bug where presigned requests wouldn't contain the same top level
operation span instrumentation because presigned requests make use of
`<Operation>::orchestrate_with_stop_point` and not
`<Operation>::orchestrate`. For IMDS and other "manually constructed"
operations, I've moved the `invoke` span instrumentation into
`Operation::invoke` to ensure we still capture that information in the
logs for those use cases.

Example output format:
```
2025-03-28T14:44:02.471068Z DEBUG S3.ListBuckets{rpc.service="S3" rpc.method="ListBuckets" sdk_invocation_id=4073472 rpc.system="aws-api"}:try_op: aws_smithy_runtime::cli
ent::orchestrator: beginning attempt #1
```

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
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.

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