TUN-5368: Log connection issues with LogLevel that depends on tunnel state

Connections from cloudflared to Cloudflare edge are long lived and may
break over time. That is expected for many reasons (ranging from network
conditions to operations within Cloudflare edge). Hence, logging that as
Error feels too strong and leads to users being concerned that something
is failing when it is actually expected.

With this change, we wrap logging about connection issues to be aware
of the tunnel state:
 - if the tunnel has no connections active, we log as error
 - otherwise we log as warning

tunnelstate/conntracker.go

@@ -0,0 +1,55 @@
+package tunnelstate
+
+import (
+	"sync"
+
+	"github.com/rs/zerolog"
+
+	"github.com/cloudflare/cloudflared/connection"
+)
+
+type ConnTracker struct {
+	sync.RWMutex
+	isConnected map[int]bool
+	log         *zerolog.Logger
+}
+
+func NewConnTracker(log *zerolog.Logger) *ConnTracker {
+	return &ConnTracker{
+		isConnected: make(map[int]bool, 0),
+		log:         log,
+	}
+}
+
+func MockedConnTracker(mocked map[int]bool) *ConnTracker {
+	return &ConnTracker{
+		isConnected: mocked,
+	}
+}
+
+func (ct *ConnTracker) OnTunnelEvent(c connection.Event) {
+	switch c.EventType {
+	case connection.Connected:
+		ct.Lock()
+		ct.isConnected[int(c.Index)] = true
+		ct.Unlock()
+	case connection.Disconnected, connection.Reconnecting, connection.RegisteringTunnel, connection.Unregistering:
+		ct.Lock()
+		ct.isConnected[int(c.Index)] = false
+		ct.Unlock()
+	default:
+		ct.log.Error().Msgf("Unknown connection event case %v", c)
+	}
+}
+
+func (ct *ConnTracker) CountActiveConns() uint {
+	ct.RLock()
+	defer ct.RUnlock()
+	active := uint(0)
+	for _, connected := range ct.isConnected {
+		if connected {
+			active++
+		}
+	}
+	return active
+}