VPP LB counters, src-ip-sticky, and frontend state aggregation

New feature: per-VIP / per-backend runtime counters
  * New GetVPPLBCounters RPC serving an in-process snapshot refreshed
    by a 5s scrape loop (internal/vpp/lbstats.go). Each cycle pulls
    the LB plugin's four SimpleCounters (next, first, untracked,
    no-server) plus the FIB /net/route/to CombinedCounter for every
    VIP and every backend host prefix via a single DumpStats call.
  * FIB stats-index discovery via ip_route_lookup (internal/vpp/
    fibstats.go); per-worker reduction happens in the collector.
  * Prometheus collector exports vip_packets_total (kind label),
    vip_route_{packets,bytes}_total, and backend_route_{packets,
    bytes}_total. Metrics source interface extended with VIPStats /
    BackendRouteStats; vpp.Client publishes snapshots via
    atomic.Pointer and clears them on disconnect.
  * New 'show vpp lb counters' CLI command. The 'show vpp lbstate'
    and 'sync vpp lbstate' commands are restructured under 'show
    vpp lb {state,counters}' / 'sync vpp lb state' to make room
    for the new verb.

New feature: src-ip-sticky frontends
  * New frontend YAML key 'src-ip-sticky' (bool). Plumbed through
    config.Frontend, desiredVIP, and the lb_add_del_vip_v2 call.
  * Reflected in gRPC FrontendInfo.src_ip_sticky and VPPLBVIP.
    src_ip_sticky, and shown in 'show vpp lb state' output.
  * Scraped back from VPP by parsing 'show lb vips verbose' through
    cli_inband — lb_vip_details does not expose the flag. The same
    scrape also recovers the LB pool index for each VIP, which the
    stats-segment counters are keyed on. This is a documented
    temporary workaround until VPP ships an lb_vip_v2_dump.
  * src_ip_sticky cannot be mutated on a live VIP, so a flipped flag
    triggers a tear-down-and-recreate in reconcileVIP (ASes deleted
    with flush, VIP deleted, then re-added). Flip is logged.

New feature: frontend state aggregation and events
  * New health.FrontendState (unknown/up/down) and FrontendTransition
    types. A frontend is 'up' iff at least one backend has a nonzero
    effective weight, 'unknown' iff no backend has real state yet,
    and 'down' otherwise.
  * Checker tracks per-frontend aggregate state, recomputing after
    each backend transition and emitting a frontend-transition Event
    on change. Reload drops entries for removed frontends.
  * checker.Event gains an optional FrontendTransition pointer;
    backend- vs. frontend-transition events are demultiplexed on
    that field.
  * WatchEvents now sends an initial snapshot of frontend state on
    connect (mirroring the existing backend snapshot), subscribes
    once to the checker stream, and fans out to backend/frontend
    handlers based on the client's filter flags. The proto
    FrontendEvent message grows name + transition fields.
  * New Checker.FrontendState accessor.

Refactor: pure health helpers
  * Moved the priority-failover selector and the (pool idx, active
    pool, state, cfg weight) → (vpp weight, flush) mapping out of
    internal/vpp/lbsync.go into a new internal/health/weights.go so
    the checker can reuse them for frontend-state computation
    without importing internal/vpp.
  * New functions: health.ActivePoolIndex, BackendEffectiveWeight,
    EffectiveWeights, ComputeFrontendState. lbsync.go now calls
    these directly; vpp.EffectiveWeights is a thin wrapper over
    health.EffectiveWeights retained for the gRPC observability
    path. Fully unit-tested in internal/health/weights_test.go.

maglevc polish
  * --color default is now mode-aware: on in the interactive shell,
    off in one-shot mode so piped output is script-safe. Explicit
    --color=true/false still overrides.
  * New stripHostMask helper drops /32 and /128 from VIP display;
    non-host prefixes pass through unchanged.
  * Counter table column order fixed (first before next) and
    packets/bytes columns renamed to fib-packets/fib-bytes to
    clarify they come from the FIB, not the LB plugin.

Docs
  * config-guide: document src-ip-sticky, including the VIP
    recreate-on-change caveat.
  * user-guide, maglevc.1, maglevd.8: updated command tree, new
    counters command, color defaults, and the src-ip-sticky field.

internal/vpp/fibstats.go

@@ -0,0 +1,87 @@
+// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>
+
+package vpp
+
+import (
+	"fmt"
+	"net"
+
+	"go.fd.io/govpp/adapter"
+	"go.fd.io/govpp/binapi/ip"
+	"go.fd.io/govpp/binapi/ip_types"
+)
+
+// routeToStatPath is the VPP stats-segment path exposing the per-FIB-entry
+// "route-to" combined counter (packets + bytes), indexed by the load-
+// balance index of each FIB entry. See lbm_to_counters in
+// src/vnet/dpo/load_balance.c.
+const routeToStatPath = "/net/route/to"
+
+// fibStatsIndex returns the FIB entry's stats_index (load_balance index)
+// for the host prefix of addr. Uses exact=0 (longest-match) so a covering
+// route is returned if there is no host-prefix entry — note this means
+// two maglev entities sharing a covering route will report identical
+// /net/route/to counters.
+func fibStatsIndex(ch *loggedChannel, addr net.IP) (uint32, error) {
+	var prefix ip_types.Prefix
+	if v4 := addr.To4(); v4 != nil {
+		prefix.Address.Af = ip_types.ADDRESS_IP4
+		copy(prefix.Address.Un.XXX_UnionData[:4], v4)
+		prefix.Len = 32
+	} else {
+		prefix.Address.Af = ip_types.ADDRESS_IP6
+		copy(prefix.Address.Un.XXX_UnionData[:], addr.To16())
+		prefix.Len = 128
+	}
+	req := &ip.IPRouteLookup{
+		TableID: 0,
+		Exact:   0,
+		Prefix:  prefix,
+	}
+	reply := &ip.IPRouteLookupReply{}
+	if err := ch.SendRequest(req).ReceiveReply(reply); err != nil {
+		return 0, fmt.Errorf("ip_route_lookup: %w", err)
+	}
+	if reply.Retval != 0 {
+		return 0, fmt.Errorf("ip_route_lookup: retval=%d", reply.Retval)
+	}
+	return reply.Route.StatsIndex, nil
+}
+
+// findCombinedCounter returns the CombinedCounterStat matching name, or
+// nil if not found or the wrong type.
+func findCombinedCounter(entries []adapter.StatEntry, name string) adapter.CombinedCounterStat {
+	for _, e := range entries {
+		if string(e.Name) != name {
+			continue
+		}
+		if s, ok := e.Data.(adapter.CombinedCounterStat); ok {
+			return s
+		}
+	}
+	return nil
+}
+
+// reduceCombinedCounter sums the (packets, bytes) CombinedCounter across
+// workers at column i, tolerating short per-worker vectors.
+func reduceCombinedCounter(s adapter.CombinedCounterStat, i int) (pkts, byts uint64) {
+	for _, thread := range s {
+		if i >= 0 && i < len(thread) {
+			pkts += thread[i][0]
+			byts += thread[i][1]
+		}
+	}
+	return pkts, byts
+}
+
+// vipKeyToIP extracts the VIP address from a vipKey's CIDR string. The
+// second return is the prefix length. Used by the scrape path to feed
+// a VIP prefix into fibStatsIndex.
+func vipKeyToIP(k vipKey) (net.IP, int, error) {
+	ip, ipnet, err := net.ParseCIDR(k.prefix)
+	if err != nil {
+		return nil, 0, err
+	}
+	ones, _ := ipnet.Mask.Size()
+	return ip, ones, nil
+}