metrics: export each session's TCP connection figures

The kernel's view of a session's TCP connection, which the status page
shows on request since the last change, could not be looked at after
the fact: a feed that crawled overnight left no record of whether the
peer's window or our own side held it up.

This change samples it at every scrape of `/metricz`: for each
established session, one `NETLINK_SOCK_DIAG` round trip, and twenty
gauges labelled by peer, `bgp_peer_tcp_*`: smoothed, least and variance
of the round-trip time; the congestion window and segment size; the
peer's receive window and ours; the send queue, the part of it not yet
sent, the receive queue and the segments in flight; retransmitted and
lost segments; the busy time and the time held up by the peer's window
and by our send buffer, cumulative, so that their rates give the share
of time a session is held up; octets sent and received; and the
delivery and pacing rates. A session the kernel has no connection for
loses its series, as does one that ends.

`SessionHandle::metric_label` is the one place a peer's metric label is
made (its name, or its address), used by the advertiser's gauge and here.

Documentation: the metrics reference lists them; the monitoring guide
gives the query for the share of time a session is held up by the
neighbor's receive window, which is the question the overnight feed
raised.

Testing: a daemon in the Linux VM with one session shows the series and
drops them when the session ends; the lab's basics phase checks the
scrape for the FRR session's round-trip time; the workspace tests pass.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/interop/lab.sh

@@ -494,6 +494,7 @@ phase_basics() {
     frr4_addr=$(api /api/peers | jq -r '.[] | select(.name == "frr4") | .peer_addr')
     api "/api/tcp?peer=$frr4_addr" | jq -e '.available and .connection.state == "established" and .connection.mss > 0' >/dev/null \
         || fail "no TCP details for frr4: $(api "/api/tcp?peer=$frr4_addr")"
+    api /metricz | grep -q "^bgp_peer_tcp_rtt_seconds{peer=\"frr4\"}" || fail "no TCP metrics for frr4"
     ok "the kernel tells of a session's TCP connection"
 }