Improved OpenTelemetry error handling (#38757)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
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**Related issue:** Resolves #38756 

- Changed to NOT mark many client errors as exceptions
- Instead, added client_error and server_error metrics that can be used
to alert on unusual error rates

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.

## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added separate metrics for distinguishing between client and server
errors, enhancing observability and monitoring capabilities.

* **Bug Fixes**
* Client request errors no longer incorrectly appear in error tracking
as exceptions; improved accuracy of error reporting to external
services.
* Adjusted logging levels for authentication and enrollment operations
to provide clearer diagnostics.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/38756-otel-client-errors

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+Improved OpenTelemetry error handling: client errors (4xx) no longer set span status to Error or appear in the Exceptions tab, following OTEL semantic conventions. Added separate metrics for client vs server errors (`fleet.http.client_errors`, `fleet.http.server_errors`) with error type attribution. Client errors are also no longer sent to APM/Sentry.