analysis: split TCP retransmissions into forward and return, and show both on the line cards

tcp.retrans.up counts retransmitted segments sent by the dialing side, tcp.retrans.down those sent by the listening side. The cards show RTT as a round trip (⇄) with forward (→) and return (←) rows for QUIC loss and TCP retransmission.

backend/src/MoShiro.Backend.Analysis/Metrics/MetricRegistry.cs

@@ -12,7 +12,7 @@ public static class MetricRegistry
     [
         LinkMetrics.Rtt, LinkMetrics.LossUp, LinkMetrics.LossDown, LinkMetrics.LossTotal,
         LinkMetrics.JitterUp, LinkMetrics.JitterDown, LinkMetrics.LossRun, LinkMetrics.LossRunUp, LinkMetrics.LossRunDown, LinkMetrics.Reorder, LinkMetrics.Dup, LinkMetrics.SamplesUdp,
-        TcpMetrics.Rtt, TcpMetrics.Slow, TcpMetrics.Stall, TcpMetrics.StallMax, TcpMetrics.Retrans, TcpMetrics.Srtt, TcpMetrics.BulkRate, TcpMetrics.BulkPeak, TcpMetrics.SamplesTcp,
+        TcpMetrics.Rtt, TcpMetrics.Slow, TcpMetrics.Stall, TcpMetrics.StallMax, TcpMetrics.Retrans, TcpMetrics.RetransUp, TcpMetrics.RetransDown, TcpMetrics.Srtt, TcpMetrics.BulkRate, TcpMetrics.BulkPeak, TcpMetrics.SamplesTcp,
         SessionMetrics.ConnectSuccess, SessionMetrics.ConnectTcp, SessionMetrics.ConnectTls,
         SessionMetrics.FailTimeout, SessionMetrics.FailReset, SessionMetrics.FailTls,
         SessionMetrics.HoldResets, SessionMetrics.HoldLifetime,
@@ -24,7 +24,7 @@ public static class MetricRegistry
     [
         new("quic", "QUIC Datagram", ["loss.up", "loss.down", "loss.total", "rtt.p50", "rtt.p95", "rtt.p99", "jitter.up.p95", "jitter.down.p95", "loss.run.up_max", "loss.run.down_max", "reorder", "dup"]),
         new("udp", "QUIC Datagram (legacy alias)", ["loss.up", "loss.down", "loss.total", "rtt.p50", "rtt.p95", "rtt.p99", "jitter.up.p95", "jitter.down.p95", "loss.run_max", "reorder", "dup"]),
-        new("tcp", "TCP", ["tcp.rtt.p50", "tcp.rtt.p95", "tcp.rtt.p99", "tcp.rtt.p999", "tcp.rtt.max", "tcp.rtt.tail", "tcp.slow", "tcp.stall.per_min", "tcp.stall_max", "tcp.retrans", "tcp.srtt.p95"]),
+        new("tcp", "TCP", ["tcp.rtt.p50", "tcp.rtt.p95", "tcp.rtt.p99", "tcp.rtt.p999", "tcp.rtt.max", "tcp.rtt.tail", "tcp.slow", "tcp.stall.per_min", "tcp.stall_max", "tcp.retrans", "tcp.retrans.up", "tcp.retrans.down", "tcp.srtt.p95"]),
         new("connect", "Connect", ["connect.success", "connect.tcp_ms.p95", "connect.tls_ms.p95", "connect.fail.timeout", "connect.fail.rst", "connect.fail.tls"]),
         new("session", "Session", ["hold.resets.per_h", "hold.lifetime_max"]),
         new("scores", "Scores", ["score.base", "score.mc", "score.hy2", "score.reality"]),