explise opened a new issue, #3219:
URL: https://github.com/apache/iggy/issues/3219
### Description
## Summary
The Go SDK currently has no way for callers to observe client-level
lifecycle events (connection state changes, auth state changes). The Rust SDK
exposes these via `Client::subscribe_events()` returning a stream of
`DiagnosticEvent`. This issue ports that capability to the Go SDK.
## Motivation
The Go TCP client already has TODO stubs in place for event emission,
indicating this was anticipated:
- `foreign/go/client/tcp/tcp_core.go:399` — `// TODO publish event`
- `foreign/go/client/tcp/tcp_core.go:461` — `// TODO event pushing logic`
- `foreign/go/client/tcp/tcp_core.go:475` — `// TODO push shutdown event.
This is groundwork for adding a high-level `IggyConsumer` abstraction to the
Go SDK (planned as a follow-up). Diagnostic events let the consumer detect
disconnects, re-join consumer groups eagerly on reconnect, and resume cleanly.
Beyond the consumer use case, the events are also useful for application-level
observability (logging connection drops, exposing health metrics, etc.).
## Scope
In scope:
1. Add a `DiagnosticEvent` type in `foreign/go/contracts/` mirroring the
Rust enum (5 variants, with a `String()` method matching the `snake_case` form
used in Rust).
2. Add `SubscribeEvents() <-chan DiagnosticEvent` to the `Client` interface
in `foreign/go/contracts/client.go`.
3. Implement a small fan-out broadcaster in the TCP client so multiple
subscribers can each receive every event independently.
4. Wire emission into the TCP client:
- `Connected` — on initial connect and on every successful auto-reconnect
(existing reconnect loop at `foreign/go/client/tcp/tcp_core.go:86-100, 351-401`)
- `Disconnected` — at the existing TODO at `tcp_core.go:399`
- `Shutdown` — at the existing TODO at `tcp_core.go:475`
- `SignedIn` — after successful `LoginUser`
- `SignedOut` — after successful `LogoutUser`
5. Integration test that subscribes, exercises connect → disconnect →
reconnect → login → logout → close, and asserts the full event sequence is
delivered.
### Component
Go SDK
### Proposed solution
_No response_
### Alternatives considered
_No response_
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