Commit Message:
Cache metadata hash on HostDescription to avoid expensive per-host comparisons during EDS updates. The hash is computed once when metadata is set (at host creation or update) and stored alongside the metadata. The comparison in updateDynamicHostList becomes a simple integer comparison instead of MessageDifferencer::Equivalent.

Additional Description:

For large clusters (~5-7k endpoints), updateDynamicHostList is a hot path where metadata comparison runs per host on each EDS update. Caching the hash eliminates the repeated serialization/comparison cost.

Benchmark results (5000 hosts, aarch64, -c opt):

Benchmark                                   Time             CPU   Iterations
bmUpdateHostListEquivalent/5000/5        4.39 ms         4.39 ms          159
bmUpdateHostListEquivalent/5000/20       14.1 ms         14.1 ms           50
bmUpdateHostListHash/5000/5              4.07 ms         4.07 ms          139
bmUpdateHostListHash/5000/20             12.6 ms         12.6 ms           49
bmUpdateHostListCachedHash/5000/5       0.001 ms        0.001 ms       553800
bmUpdateHostListCachedHash/5000/20      0.001 ms        0.001 ms       556835

• Equivalent: baseline (reflection-based proto comparison)
• Hash: recomputing hash each time (~8% faster)
• CachedHash: pre-computed hash comparison (~4000x faster)

Risk Level: Low
Testing: Benchmark added (test/common/upstream/metadata_comparison_benchmark.cc)
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
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Request variables

Key	Value
ref	52bdfcb
sha	9d302b6
pr	43351
base-sha	af4e593
actor	@wdauchy
message	upstream: cache metadata hash for O(1) comparison in EDS updates...
started	1771081447.654611
target-branch	main
trusted	false

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