[Proposal] Stop ctx.Value from being an implicit RPC parameter source

[Alanxtl]

Parent issue: #3595

Problem

The client invocation path currently treats ctx.Value(constant.AttachmentKey) as an implicit source of RPC attachments. In client.generateInvocation, values stored in the Context are copied into Invocation.attachments and may then be serialized as Dubbo attachments, Triple HTTP headers, or Dubbo3/gRPC metadata.

This means a call such as:

ctx = context.WithValue(ctx, constant.AttachmentKey, map[string]any{
    "tenant-id": "tenant-a",
})
client.CallUnary(ctx, req, resp, "GetUser")

has hidden RPC inputs that are not visible in the call options or business request. The same mechanism can also affect RPC control fields such as timeout and retries.

The server side may expose inbound metadata both through ctx.Value(constant.AttachmentKey) and through Invocation.attachments, which further blurs the boundary between request lifecycle context and transport metadata.

Goals

• Make outbound RPC metadata explicit at the call site.
• Keep context.Context focused on cancellation, deadlines, tracing, and local request-scoped state.
• Preserve existing users through a documented compatibility path.
• Ensure explicit metadata has deterministic precedence over legacy metadata.
• Prevent a metadata timeout from extending the caller's Context deadline.

[Alanxtl]

我觉得还是不改比较好 用户修改成本比较高 又没啥收益