Fix Arista failures in telemetry_interface_last_change_test (gNMI-1.25)

[smharis-arista]

Issue#1
Add subinterface_0_state_unsupported deviation for Arista as a few state telemetry is not populated on the default (index 0) subinterface (both LAG and Ethernet). The deviation is defined broadly to cover any unsupported state on subinterface 0, but in this PR it is used specifically to skip last-change validation while parent interface validation continues -- Please see https://partnerissuetracker.corp.google.com/issues/456175792

Issue#2
Fix LAGMemberFlap and OTGLAGFlap timeout by expecting LOWER_LAYER_DOWN oper-status on Arista when the LAG goes down due to member port disable or link loss. The oper-status remains DOWN when LAG itself is shut.

Arista's default behavior is

1. When the only member interface of a port-channel (LAG) is shut down, the operation status (operStatus) on the port-channel interface is set to lowerLayerDown.
2. When The LAG itself is shut down, the operation status (operStatus) on the port-channel interface is set to Down.

The existing vendor checks are not touched and abstracted into a single logic since they don't seem to align properly with Arista's default behavior.

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[gemini-code-assist]

Summary of Changes

Hello, 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 addresses specific test failures observed on Arista devices within the telemetry_interface_last_change_test suite. It introduces a new platform deviation to account for Arista's behavior regarding state population on implicitly created subinterface 0, allowing the test to conditionally bypass validation for this specific scenario. Additionally, it refines the expected operational status for LAG interfaces during flap events on Arista, ensuring test accuracy when member ports are disabled or links are lost.

Highlights

• Arista Subinterface 0 Deviation: Introduced a new deviation subinterface_0_state_unsupported for Arista devices, acknowledging that they may not populate state on implicitly created subinterface 0.
• Conditional Last-Change Validation: Modified telemetry_interface_last_change_test to conditionally skip last-change validation for subinterface 0 on Arista devices when the new deviation is enabled.
• LAG Oper-Status Adjustment: Updated LAG flap tests (LAGMemberFlap, OTGLAGFlap) for Arista to expect LOWER_LAYER_DOWN as the operational status when a LAG goes down due to member port disablement or link loss.

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[gemini-code-assist]

Code Review

This pull request implements a new deviation, subinterface_0_state_unsupported, to accommodate devices that do not report state for implicitly created subinterface 0. Corresponding updates were made to the telemetry interface last change tests to conditionally skip subinterface checks. Additionally, vendor-specific logic was added for Arista LAG flap tests. The reviewer suggested abstracting the vendor-specific operational status logic to enhance maintainability and promote reusability across different platforms.

[gemini-code-assist]

The condition for Arista LAG flap timeout is specific to LAGMemberFlap and OTGLAGFlap. Consider if this logic should be abstracted or if there is a more generic way to handle vendor-specific oper-status expectations for LAG flaps to improve maintainability, as constants and logic applicable to multiple vendors should be kept general to promote reusability.

References

1. Constants that may be applicable to multiple vendors in the future should be kept general, even if they are currently used in a vendor-specific context, to promote reusability.