apollo_network_benchmark: add get_tcp_stats function

[sirandreww-starkware]

Note

Low Risk
Adds a new, currently unused /proc parsing helper; no behavior changes unless it is later integrated into the metrics pipeline.

Overview
Adds a new Linux-only helper get_tcp_stats() in system_metrics.rs that parses /proc/net/snmp to extract TCP OutSegs and RetransSegs, with warning logs on read/parse/format issues.

The function is currently unused (guarded by #[allow(dead_code)]), so runtime behavior of the benchmark node is unchanged until it’s wired into metric collection.

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[reviewable-StarkWare]

This change is 

[sirandreww-starkware]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• apollo_network_benchmark: add collect_tcp_metrics and integrate into monitoring loop #11563
• apollo_network_benchmark: add get_tcp_stats function #11562 👈 (View in Graphite)
• apollo_network_benchmark: add TCP metrics definitions #11561
• apollo_network_benchmark: implement RoundRobin broadcasting logic #11560 : 2 other dependent PRs (#12722 , #13288 )
• apollo_network_benchmark: add seconds_since_epoch helper function #11559
• apollo_network_benchmark: add RoundRobin mode to enum and round_duration_seconds arg #11558
• apollo_network_benchmark: add message index detection mechanism #11557
• apollo_network_benchmark: add MessageIndexTracker struct #11556
• apollo_network_benchmark: add ReveresedSqmr receiver and registration #11555
• apollo_network_benchmark: add ReveresedSqmrSender struct and implementation #11554
• apollo_network_benchmark: add ReveresedSqmr variant to NetworkProtocol enum #11553
• apollo_network_benchmark: add Sqmr variant to MessageReceiver and registration #11552
• apollo_network_benchmark: add Sqmr variant to MessageSender with send logic #11551
• apollo_network_benchmark: add Sqmr variant to NetworkProtocol enum #11550
• apollo_network_benchmark: add SQMR protocol constant #11549
• apollo_network_benchmark: added ping latency metrics #11548
• apollo_network_benchmark: added network event metrics #11547
• apollo_network_benchmark: added SQMR session metrics #11546
• apollo_network_benchmark: added broadcast topic metrics #11545
• apollo_network_benchmark: added network metrics to network manager #11544
• apollo_network_benchmark: added broadcast metrics tracking #11543
• apollo_network_benchmark: added conditional broadcasting based on mode #11542
• apollo_network_benchmark: added broadcasting mode selection #11541
• apollo_network_benchmark: added message broadcasting task #10734 : 1 other dependent PR (#8303 )
• apollo_network_benchmark: added register metrics #10660 : 1 other dependent PR (#10661 )
• main

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