use metadata to specify use of default identity

[havetisyan]

Description

gcp has a limit of 6 chars min for service name but some teams might already have smaller service name already used in their production workloads. ZTS already provides support for providing such identites but sia agent assumes that a multiple service is in play and uses the service specific identity when fetching the attestation token.

For example, the gcp service account is configured as zts-gcp, but we want an identity on the box for a service called zts. We drop a sia_config file as expected with the value of zts. Now, when sia fetches the attestation data it uses the service account name in the token as zts. However, with this feature if the instance has the "defaultServiceIdentity" attribute with the value of zts, then when processing, the sia agent will use the default identity for the zts service which in case will be the zts-gcp identity.

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello @havetisyan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant improvement to how the SIA agent handles identity token attestation on Google Cloud Platform. It provides a mechanism to explicitly define a 'default service identity' through instance metadata. This change addresses scenarios where service names might not align with GCP's naming conventions, ensuring that the SIA agent can correctly retrieve identity tokens without resorting to more complex impersonation flows, thereby increasing flexibility and compatibility for existing workloads.

Highlights

• Flexible Identity Token Retrieval: The SIA agent's logic for fetching attestation tokens has been enhanced to support using a 'default service identity' specified via instance metadata. This allows services with names that might not conform to GCP's minimum length requirements (e.g., less than 6 characters) to still utilize a pre-configured identity.
• Metadata-driven Default Identity: A new instance attribute, 'defaultServiceIdentity', can now be configured. If the requested service name matches this attribute's value, the agent will directly fetch the identity token from the metadata server, similar to when the service name matches the instance's primary service account.
• Expanded Test Coverage: New test cases have been added to validate the new logic, including scenarios for direct metadata token retrieval when using the default service identity, and comprehensive error handling tests for various failure conditions during token fetching.

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to use the instance's default identity for a service by specifying a defaultServiceIdentity metadata attribute. This is useful when the desired service name doesn't match the GCP service account name. The changes in attestation.go implement this logic, and new tests are added in attestation_test.go to cover the new functionality. My review focuses on improving the robustness of the new logic and correcting issues in the new tests. I've suggested handling a potential edge case with empty service names and improving error visibility in the main logic. For the tests, I've pointed out a logic bug that prevents a test from failing correctly and recommended using test-specific logging for cleaner output. Overall, the changes are good and the tests are comprehensive, but these adjustments will improve the quality and maintainability.