gep: GEP-3440 - Gateway API Support for gRPC Retries

[shadialtarsha]

What type of PR is this?

/kind gep

What this PR does / why we need it:
Proposes configurations for gRPC retries within GRPCRotue.

Which issue(s) this PR fixes:

Fixes #3440

Does this PR introduce a user-facing change?:

Adds a new field `retry` to `GRPCRouteRule` to allow configuring retries for unsuccessful gRPC requests. 

[linux-foundation-easycla]

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• ✅ login: shadialtarsha / name: Shadi Altarsha (5b821c1, 54e5c50, 7cecf09, 0357b13, e435f1a, 072e76e)

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[shadialtarsha]

@mikemorris already set me up for success here with his HTTP retries GEP. So this GEP is heavily inspired by his work.

I assume this GEP might depend on the output of #3219 or at least the conformance tests need the timeout to test the backoff not exceeding the timeout.

By the way this is my first PR so I hope I followed the process correctly 😅

[howardjohn]

How is this enforced in the API specification?

[shadialtarsha]

Thanks for calling that out. The API doesn't have a way to enforce this non-goal.

I am thinking of three ways to do that:

1. Adding a restriction in the API documentation clarifying that retries apply only to unary calls, with a potential future option to expand to streaming. Something among the line as:

// Note: **Retries are supported only for unary gRPC calls.**
// Implementations MUST NOT apply retries to streaming or bidirectional
// gRPC calls, as these types of calls are stateful and retrying them
// could result in data loss or duplication.

2. Explicit Field: Add a UnaryOnly field (e.g., UnaryOnly bool) that makes it clear retries are restricted to unary calls.
3. Remove this restriction and let users choose whether to apply retries on any gRPC call type.

Would like to hear your thoughts on this.

[robscott]

@shadialtarsha Thanks for this PR! We're currently in our release scoping phase for v1.3. To have this considered for scope in v1.3, please propose it in #3403.