[processor/k8sattributesprocessor] Add pod_delete_grace_period configuration option

[giuliano-sider]

Description

This PR adds the pod_delete_grace_period configuration option to the k8sattributes processor, making the cache eviction grace window for deleted pods configurable.

By default, after receiving a pod deletion event, the processor keeps the pod's metadata in its lookup cache for a short duration (120s) before eviction. This ensures that delayed spans, metrics, or logs originating from the pod can still be correctly enriched. With this change, users can tune this duration to match their environment's constraints (e.g. reducing memory overhead by lowering the duration, or setting it to 0s to evict immediately).

Important

Stacked PR: This PR is stacked on top of [processor/k8sattributesprocessor] Add watch_sync_period config option. Only the changes in the second commit (Add pod_delete_grace_period config option...) are new to this PR.

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Link to tracking Issue

The issue of configurability (and its longstanding TODO) are brought up in #48110.

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Testing & Verification

Automated Tests

• Added unit test coverage in config_test.go ensuring custom pod_delete_grace_period values (e.g., 30s) are successfully loaded, parsed, and validated.
• Added unit test coverage verifying that negative durations (e.g., -30s) correctly fail configuration validation.
• All tests compiled and passed successfully:

  go test -v ./...

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Documentation

• Added a description of the config in the README.
• Added an entry to the user changelog.

[github-actions]

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