cmd/{k8s-proxy,containerboot,k8s-operator},kube: add health check and metrics endpoints for k8s-proxy (#16540)

* Modifies the k8s-proxy to expose health check and metrics
endpoints on the Pod's IP.

* Moves cmd/containerboot/healthz.go and cmd/containerboot/metrics.go to
  /kube to be shared with /k8s-proxy.

Updates #13358

Signed-off-by: David Bond <davidsbond93@gmail.com>

kube/health/healthz.go

@@ -0,0 +1,84 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !plan9
+
+// Package health contains shared types and underlying methods for serving
+// a `/healthz` endpoint for containerboot and k8s-proxy.
+package health
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"sync"
+
+	"tailscale.com/client/local"
+	"tailscale.com/ipn"
+	"tailscale.com/kube/kubetypes"
+	"tailscale.com/types/logger"
+)
+
+// Healthz is a simple health check server, if enabled it returns 200 OK if
+// this tailscale node currently has at least one tailnet IP address else
+// returns 503.
+type Healthz struct {
+	sync.Mutex
+	hasAddrs bool
+	podIPv4  string
+	logger   logger.Logf
+}
+
+func (h *Healthz) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	h.Lock()
+	defer h.Unlock()
+
+	if h.hasAddrs {
+		w.Header().Add(kubetypes.PodIPv4Header, h.podIPv4)
+		if _, err := w.Write([]byte("ok")); err != nil {
+			http.Error(w, fmt.Sprintf("error writing status: %v", err), http.StatusInternalServerError)
+		}
+	} else {
+		http.Error(w, "node currently has no tailscale IPs", http.StatusServiceUnavailable)
+	}
+}
+
+func (h *Healthz) Update(healthy bool) {
+	h.Lock()
+	defer h.Unlock()
+
+	if h.hasAddrs != healthy {
+		h.logger("Setting healthy %v", healthy)
+	}
+	h.hasAddrs = healthy
+}
+
+func (h *Healthz) MonitorHealth(ctx context.Context, lc *local.Client) error {
+	w, err := lc.WatchIPNBus(ctx, ipn.NotifyInitialNetMap)
+	if err != nil {
+		return fmt.Errorf("failed to watch IPN bus: %w", err)
+	}
+
+	for {
+		n, err := w.Next()
+		if err != nil {
+			return err
+		}
+
+		if n.NetMap != nil {
+			h.Update(n.NetMap.SelfNode.Addresses().Len() != 0)
+		}
+	}
+}
+
+// RegisterHealthHandlers registers a simple health handler at /healthz.
+// A containerized tailscale instance is considered healthy if
+// it has at least one tailnet IP address.
+func RegisterHealthHandlers(mux *http.ServeMux, podIPv4 string, logger logger.Logf) *Healthz {
+	h := &Healthz{
+		podIPv4: podIPv4,
+		logger:  logger,
+	}
+	mux.Handle("GET /healthz", h)
+	return h
+}