metadata: replace added [][]string with O(1) delta linked list

[notandruu]

Problem

AppendToOutgoingContext is O(N) per call because it allocates a new
[][]string of length N+1 and copies all N prior slices on every
invocation. A call chain with N sequential appends costs O(N²) in total
allocation and copy work. This is visible in production as latency spikes
in metadata-heavy RPCs.

Fixes #8860.

Solution

Replace rawMD.added [][]string with a singly-linked list of deltaKV
nodes. Each AppendToOutgoingContext call allocates exactly one node and
one flattened kv slice — O(1) regardless of chain depth.

FromOutgoingContext and fromOutgoingContextRaw traverse the chain
(newest-first), collect into a temporary slice, then iterate in reverse
to preserve FIFO ordering, keeping the read path O(N) and maintaining
backward-compatible return types for internal callers in transport and
stream code.

Benchmarks

Apple M3 Max, go1.26.3:

BenchmarkAppendToOutgoingContext (accumulating context, num=10):
  before: 3,263,813 ns/op  (quadratic growth)
  after:      1,646 ns/op  (1982× faster)

BenchmarkFromOutgoingContext:
  before: 544.3 ns/op
  after:  212.4 ns/op  (2.6× faster)

BenchmarkAppendToOutgoingContextN (fresh context, N sequential appends):
  N= 1:   87 ns/op,  160 B/op,   4 allocs
  N= 5:  446 ns/op,  800 B/op,  20 allocs
  N=10:  872 ns/op, 1600 B/op,  40 allocs
  N=50: 4373 ns/op, 8000 B/op, 200 allocs
  (linear: ~87·N ns/op)

All existing metadata tests pass.

RELEASE NOTES:

• metadata: AppendToOutgoingContext is now O(1) per call instead of O(N), eliminating quadratic allocation overhead in metadata-heavy RPC call chains.

[linux-foundation-easycla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.25%. Comparing base (6602080) to head (7f6f7d3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9129      +/-   ##
==========================================
+ Coverage   83.20%   83.25%   +0.04%     
==========================================
  Files         414      414              
  Lines       33489    33496       +7     
==========================================
+ Hits        27865    27887      +22     
+ Misses       4214     4201      -13     
+ Partials     1410     1408       -2     

Files with missing lines	Coverage Δ
metadata/metadata.go	94.78% <100.00%> (+2.19%)	⬆️

... and 17 files with indirect coverage changes

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[notandruu]

Could a maintainer add a Type: label to unblock PR Validation? Happy to make any changes needed.