Add OpenMetrics exporter (with QUIC stats)

[avevad]

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[EmelyanenkoK]

@codex please review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fix mean RTT aggregation to use other entry

The weighted average for mean_rtt mistakenly uses impl_stats.mean_rtt for both terms, so combining two entries always ignores other.impl_stats.mean_rtt. As a result, server-level RTT stats are wrong whenever more than one connection contributes (the aggregate stays at the current entry’s RTT). This affects all aggregated stats emitted via Stats::Entry::operator+.

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[chatgpt-codex-connector]

Log period-1 stats from the correct bucket

When emitting PERIOD 1 logs, the code reads std::get<2>(period0_stats) instead of period1_stats, so the PERIOD 1 summary/top-5 is always a duplicate of PERIOD 0. This makes the period-1 monitoring output misleading and hides its intended time window.

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[github-actions]

I can’t review the PR changes because commit 5f8fe389ec6b30e45298327d665a61e820bc869d isn’t present in this repo (the range 48e1009494d5f36f4febfbb9dfa7193b5eaff4fd...5f8fe389ec6b30e45298327d665a61e820bc869d is invalid here). Please provide a repo state that includes that commit (e.g., update the checkout or give an alternate range/patch), and I’ll review only those changes.

[github-actions]

I can’t review this PR because commit 22d70654ccaadff704ed22835ae5c7961caf42db isn’t present in the local repo, so the range 48e1009494d5f36f4febfbb9dfa7193b5eaff4fd...22d70654ccaadff704ed22835ae5c7961caf42db is invalid. Please provide a range that exists locally or share the patch/branch containing that commit.

[github-actions]

• quic/quic-server.h:69: weighted mean RTT uses impl_stats.mean_rtt for both sides; other.impl_stats.mean_rtt is ignored, so the average never reflects the RHS. This should be other.impl_stats.mean_rtt.
• quic/quic-server.h:62-79: Stats::summary starts with Entry{} where total_conns defaults to 1, so every summary accumulation is off by +1. Consider defaulting total_conns to 0 for summaries (and explicitly set per-conn entries to 1).
• quic/quic-sender.cpp:309-311: period 1 logging uses std::get<2>(period0_stats) instead of period1_stats, so PERIOD 1 output is actually period 0 data.

[github-actions]

• quic/quic-sender.cpp:309: stats1 is pulled from period0_stats instead of period1_stats, so PERIOD 1 logs always repeat PERIOD 0 data.
• quic/quic-sender.cpp:245: by_cid_[id] assumes the CID exists; if a server connection is present but the sender map hasn’t been updated (or has been removed), this will insert a null shared_ptr and dereference it. Safer to find and skip/guard missing CIDs.
• quic/quic-sender.cpp:202-208: Stats::operator- only computes per-path deltas for paths present in other. New paths since the last sample get dropped entirely, so period deltas miss new connections. Consider iterating per_path and subtracting when present in other, or merging both maps.
• quic/quic-pimpl.cpp:397-406 vs 505-509: sids_encountered increments only for remote-initiated bidi streams. Locally opened streams via open_stream() are never counted, so total_sids underreports for outbound traffic.