Prometheus: add VPP, LB sync, and gRPC metrics; expand docs

New metrics plus the corresponding documentation for everything that's
accumulated since the last Prometheus pass.

internal/metrics/metrics.go
- New VPPSource interface (IsConnected, VPPInfo) plus a metrics-local
  VPPInfo struct that mirrors vpp.Info. Decoupling via interface +
  struct-mirror keeps the dependency direction one-way (vpp → metrics),
  so vpp can import metrics to update inline counters without a cycle.
- New Collector gauges scraped on demand: maglev_vpp_connected,
  maglev_vpp_uptime_seconds (from /sys/boottime), maglev_vpp_connected_seconds
  (time since maglevd connected), and maglev_vpp_info (static 1-gauge
  carrying version, build_date, and pid as labels).
- New inline counters:
  - maglev_vpp_api_total{msg, direction, result} — bumped from the
    loggedChannel wrapper on every VPP binary-API send/recv. Gives full
    visibility into what maglevd is doing with VPP, broken down by
    message name, direction (send/recv), and result (success/failure).
  - maglev_vpp_lbsync_total{scope, kind} — bumped from the reconciler
    at the end of each SyncLBStateAll/SyncLBStateVIP run. kind ∈
    {vip_added, vip_removed, as_added, as_removed, as_weight_updated};
    scope ∈ {all, vip}. Zero-valued kinds are not emitted so noise
    stays low.
- Register() signature now takes a VPPSource (may be nil) alongside
  the existing StateSource.

internal/vpp/client.go
- New VPPInfo() (metrics.VPPInfo, bool) shim method on *Client that
  satisfies metrics.VPPSource. Returns (_, false) when disconnected so
  the collector skips the vpp_* gauges cleanly.

internal/vpp/apilog.go
- The loggedChannel's SendRequest / SendMultiRequest / ReceiveReply
  paths now call metrics.VPPAPITotal.WithLabelValues(...).Inc() in
  addition to slog.Debug. Since every VPP API call in the codebase
  must go through loggedChannel (NewAPIChannel is unexported), this
  one instrumentation point catches everything.

internal/vpp/lbsync.go
- New recordSyncStats(scope, st) helper called once at the end of
  SyncLBStateAll and SyncLBStateVIP to bump maglev_vpp_lbsync_total.
  Zero-valued stats are skipped.

cmd/maglevd/main.go
- Added github.com/grpc-ecosystem/go-grpc-middleware/providers/prometheus
  for the standard gRPC server metrics (grpc_server_started_total,
  grpc_server_handled_total, grpc_server_handling_seconds, etc.,
  labelled by service/method/type/code).
- Constructs grpcprom.NewServerMetrics(WithServerHandlingTimeHistogram())
  before creating the grpc.Server, installs it as UnaryInterceptor +
  StreamInterceptor, then calls InitializeMetrics(srv) after service
  registration so every method appears at 0 on the first scrape
  instead of materialising lazily on first RPC.
- Passes the vppClient (or nil) as a metrics.VPPSource to
  metrics.Register so the vpp_* gauges are emitted when integration
  is enabled and silently omitted otherwise.

docs/user-guide.md
- New 'Prometheus metrics' section in the maglevd chapter,
  tabulating every metric family: backend state gauges, probe
  counters/histogram, transition counters, the new VPP gauges and
  counters, and the standard gRPC server metrics.
- 'show frontends <name>' description updated to mention the two
  weight columns ('weight' = configured from YAML, 'effective' =
  state-aware after pool-failover logic).
- Pause / disable descriptions clarified: transition history is
  preserved across these operator actions.

docs/healthchecks.md
- New 'Static (no-healthcheck) backends' section explaining that
  backends without a healthcheck use rise/fall=1, fire a synthetic
  passing probe immediately on startup (no 30s wait), and idle at
  30s between iterations thereafter.
- New 'Pool failover' section documenting the priority-tier model,
  the active-pool definition, when promotion happens, cascading to
  further tiers, and graceful drain on demotion. Points readers at
  'maglevc show frontends <name>' as the inspection interface.

docs/config-guide.md
- healthcheck field doc now describes static-backend behavior and
  cross-references healthchecks.md.
- pools field doc now explains failover semantics at a high level
  and cross-references the detailed healthchecks.md section.

docs/config-guide.md

@@ -239,8 +239,10 @@ multiple frontends.
 
 *   ***address***: Required. The IPv4 or IPv6 address of this backend server.
 *   ***healthcheck***: The name of a health check defined in the `healthchecks` section.
-    When empty or omitted, no probing is performed and the backend is assumed permanently
-    healthy. This is useful for backends that are always available or managed by other means.
+    When empty or omitted, the backend is static: no probing is performed and the backend
+    enters `StateUp` immediately on startup (via a synthetic pass, rise/fall forced to 1/1).
+    This is useful for backends that are always available or managed by other means. See
+    [healthchecks.md](healthchecks.md) for details on the static-backend behavior.
 *   ***enabled***: A boolean controlling whether this backend participates in any frontend.
     When `false`, the backend is excluded entirely and no probe goroutine is started.
     Defaults to `true`.
@@ -280,8 +282,12 @@ ordered list of backend pools. The gRPC API exposes frontends by name.
     frontend port is independent of the healthcheck port: a frontend on port 443 may use
     a healthcheck that probes port 80.
 *   ***pools***: Required. A non-empty ordered list of pool objects. Pools express priority:
-    the first pool is preferred; subsequent pools act as fallbacks. All backends across all
-    pools in a frontend must have addresses of the same address family (all IPv4 or all IPv6).
+    the first pool is preferred; subsequent pools act as fallbacks. When every backend in
+    pool[0] leaves `StateUp` (down, paused, disabled, or not yet probed), pool[1] is
+    automatically promoted — its up backends take over serving traffic. The promotion
+    cascades across further tiers. See [healthchecks.md](healthchecks.md#pool-failover)
+    for the full failover semantics. All backends across all pools in a frontend must
+    have addresses of the same address family (all IPv4 or all IPv6).
 
 Each pool has: