feat: add resend attribute to span tracing

[ldetmer]

This adds attribute for HTTP_REQUEST_RESEND_COUNT to span tracing. This only gets populated in case of a retry on the client side.

no resend attribute on first attempt
example retry 1
example retry 4

[gemini-code-assist]

Summary of Changes

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This pull request enhances the BigQuery client's OpenTelemetry tracing capabilities by introducing a new HTTP_REQUEST_RESEND_COUNT attribute. This attribute will be populated on HTTP request spans only when a client-side retry occurs, providing valuable insight into the retry behavior of requests. The change involves tracking retry attempts using a ThreadLocal variable and integrating this information into the tracing initializer.

Highlights

• Retry Attempt Tracking: Introduced a ThreadLocal mechanism within BigQueryRetryAlgorithm to accurately track the current retry attempt count for client-side operations.
• OpenTelemetry Span Attribute: Added a new HTTP_REQUEST_RESEND_COUNT attribute to OpenTelemetry spans, which is populated with the retry attempt count when an HTTP request is retried.
• Attribute Reset Logic: Implemented logic to reset the ThreadLocal retry attempt count after each request to prevent state leakage across different requests on the same thread.
• Comprehensive Testing: Included new unit tests across multiple files to validate that the HTTP_REQUEST_RESEND_COUNT attribute is correctly set during retries and appropriately absent when tracing is disabled or no retry occurs.

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to track and report HTTP request resend counts in OpenTelemetry traces for BigQuery operations, utilizing a ThreadLocal variable to store the current retry attempt. The primary feedback highlights a potential ThreadLocal state leakage in BigQueryRetryAlgorithm if a retry sequence fails completely, which could lead to incorrect tracing data for subsequent requests on the same thread. Additionally, the newly added tests that manually set the ThreadLocal state lack try-finally blocks for cleanup, making them fragile and prone to leaking state between tests.