feat(instrumentation): add span attr for Bedrock prompt caching

[thisthat]

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Follow up for the suggestion made by @nirga: #2788 (review)

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Important

Adds span attribute for prompt caching in Bedrock instrumentation and tests for cache read/write scenarios.

• Behavior:
  • Adds gen_ai.prompt_caching span attribute in prompt_caching_handling() in prompt_caching.py.
  • Sets attribute to "read" or "write" based on cache token counts.
• Tests:
  • Adds test_prompt_cache in test_anthropic.py to verify span attributes for cache reads and writes.
  • Includes test_prompt_cache.yaml for VCR testing with specific request/response scenarios.

^{This description was created by for ee0a081. It will automatically update as commits are pushed.}

[ellipsis-dev]

❌ Changes requested. Reviewed everything up to ee0a081 in 2 minutes and 13 seconds

More details

• Looked at 226 lines of code in 3 files
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• Skipped posting 2 drafted comments based on config settings.

1. packages/opentelemetry-instrumentation-bedrock/opentelemetry/instrumentation/bedrock/prompt_caching.py:29

• Draft comment:
  If both READ and WRITE headers are present, the second set_attribute call will override the first. If this behavior is intended, please add a comment clarifying it.
• Reason this comment was not posted:
  Confidence changes required: 33% <= threshold 50%
  None

2. packages/opentelemetry-instrumentation-bedrock/tests/traces/test_anthropic.py:325

• Draft comment:
  Unreachable code: The call to prompt_caching_call() after the return in the inner function is never executed. Please remove or reposition it.
• Reason this comment was not posted:
  Marked as duplicate.

Workflow ID: wflow_zIMS26lgZOTeLwWl

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[nirga]

Thanks @thisthat!