fix(health): keep gNMI STREAM request streams open

[jayzhudev]

Keep the client stream open after sending the initial gNMI SubscriptionList for SAMPLE and ON_CHANGE STREAM subscriptions. This allows the gNMI target to return sync_response and subsequent telemetry without treating request-stream EOF as RPC termination.

The gNMI specification does not explicitly require the client stream to remain open. However, many gNMI target implementations interpret request-stream EOF as termination of the subscription. Keeping the client stream open is a safe compatibility fix.

Related issues

Fixes #4754

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

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📥 Commits

Reviewing files that changed from the base of the PR and between b0439c2 and 1223d32.

📒 Files selected for processing (1)

• crates/health/src/collectors/nvue/gnmi/client.rs

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Summary by CodeRabbit

• Bug Fixes
  • Improved gNMI subscription reliability by keeping SAMPLE and ON_CHANGE subscription streams open after the initial request.
  • Helps ensure ongoing health data updates continue to be received correctly.

Walkthrough

The gNMI client now keeps SAMPLE and ON_CHANGE subscription request streams open after sending the initial request. This prevents the client request side from reaching EOF during long-lived subscriptions.

Changes

gNMI subscription lifecycle

Layer / File(s)	Summary
Persistent subscription request streams crates/health/src/collectors/nvue/gnmi/client.rs	The client uses StreamExt to send each initial SAMPLE and ON_CHANGE request once, then keep both request streams pending.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the gNMI STREAM request-stream fix and matches the primary change.
Description check	✅ Passed	The description accurately explains the gNMI stream-closure bug, the compatibility fix, and the related issue.
Linked Issues check	✅ Passed	The changes keep SAMPLE and ON_CHANGE request streams open, directly addressing issue #4754 and its stated requirements.
Out of Scope Changes check	✅ Passed	The changes are limited to keeping the two gNMI STREAM request streams open and are within issue #4754 scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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