attributes: Replace internal map with linked list (#8933) Fixes: #8921 This PR replaces the underlying `map[any]any` implementation in the `Attributes` struct with a singly linked list. Because `Attributes` are immutable, the previous implementation required a complete allocation and map copy every time `WithValue` was called. Since this data structure is primarily used in the control plane and typically holds a small number of values (10-20 at most), optimizing for memory is significantly more valuable than optimizing for lookup speed. By switching to a persistent linked-list , appending a new value becomes a highly efficient operation. `WithValue` now simply prepends a new node that points to the previous state, entirely eliminating the need to copy existing elements. Shadowing is handled naturally by returning the first match found during traversal. This approach is very similar to the implementation of [context.WithValue](https://cs.opensource.google/go/go/+/refs/tags/go1.26.0:src/context/context.go;l=727;drc=1a53ce9734c0b2a3e2a9814e75949ea77a978143). ### Complexity Comparison The following table breaks down the time and memory complexity of the `Attributes` operations, where $N$ is the number of keys. | Operation | Previous Time | Previous Space | New Time | New Space | Notes on New Implementation | | :--- | :--- | :--- | :--- | :--- | :--- | | **`New`** | $O(1)$ | $O(1)$ | $O(1)$ | $O(1)$ | Allocates a single head node. | | **`WithValue`** | $O(N)$ | $O(N)$ | $O(1)$ | $O(1)$ | **Highly Optimized:** Merely prepends a new node to the existing list instead of copying a full map. | | **`Value`** (Lookup) | $O(1)$ | $O(1)$ | $O(N)$ | $O(1)$ | Requires a linear scan. For $N \le 20$, this overhead is negligible. | | **`Equal`** | $O(N)$ | $O(1)$ | $O(N)$ | $O(N)$ | Allocates temporary maps to track shadowed keys and compare list contents. | ### Benchmark The benchmark added compares the memory usage for storing 10 attributes in 50 endpoints. It shows a 86% reduction is memory utilization. ```sh goos: linux goarch: amd64 pkg: google.golang.org/grpc/attributes cpu: Intel(R) Xeon(R) CPU @ 2.60GHz │ old.txt │ new.txt │ │ sec/op │ sec/op vs base │ WithValue-48 168.26µ ± 0% 15.53µ ± 1% -90.77% (p=0.000 n=10) │ old.txt │ new.txt │ │ B/op │ B/op vs base │ WithValue-48 200.00Ki ± 0% 23.44Ki ± 0% -88.28% (p=0.000 n=10) │ old.txt │ new.txt │ │ allocs/op │ allocs/op vs base │ WithValue-48 1700.0 ± 0% 500.0 ± 0% -70.59% (p=0.000 n=10) ``` RELEASE NOTES: * attributes: Optimize memory utilization from $O(n)$ to $O(1)$.
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