feat(healthcheck): add TLS SNI header to request when needed

Add the Server Name Indication (SNI) field to healthcheck requests, if
present in the target config.

SNI handling is already present for proxying resources, but this has
not been implemented for healthcheck requests yet until this commit.

In order to facilitate this, this commit moves the client instantiation
to when the healthcheck is performed, rather than as a part of the
monitor init.

healthcheck/healthcheck.go

@@ -48,6 +48,7 @@ type Config struct {
 	Headers           map[string]string `json:"hcHeaders"`
 	Method            string            `json:"hcMethod"`
 	Status            int               `json:"hcStatus"` // HTTP status code
+	TLSServerName     string            `json:"hcTlsServerName"`
 }
 
 // Target represents a health check target with its current status
@@ -70,7 +71,6 @@ type Monitor struct {
 	targets     map[int]*Target
 	mutex       sync.RWMutex
 	callback    StatusChangeCallback
-	client      *http.Client
 	enforceCert bool
 }
 
@@ -78,21 +78,10 @@ type Monitor struct {
 func NewMonitor(callback StatusChangeCallback, enforceCert bool) *Monitor {
 	logger.Debug("Creating new health check monitor with certificate enforcement: %t", enforceCert)
 
-	// Configure TLS settings based on certificate enforcement
-	transport := &http.Transport{
-		TLSClientConfig: &tls.Config{
-			InsecureSkipVerify: !enforceCert,
-		},
-	}
-
 	return &Monitor{
 		targets:     make(map[int]*Target),
 		callback:    callback,
 		enforceCert: enforceCert,
-		client: &http.Client{
-			Timeout:   30 * time.Second,
-			Transport: transport,
-		},
 	}
 }
 
@@ -388,6 +377,17 @@ func (m *Monitor) performHealthCheck(target *Target) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(target.Config.Timeout)*time.Second)
 	defer cancel()
 
+	client := &http.Client{
+		Transport: &http.Transport{
+			TLSClientConfig: &tls.Config{
+				// Configure TLS settings based on certificate enforcement
+				InsecureSkipVerify: !m.enforceCert,
+				// Use SNI TLS header if present
+				ServerName: target.Config.TLSServerName,
+			},
+		},
+	}
+
 	req, err := http.NewRequestWithContext(ctx, target.Config.Method, url, nil)
 	if err != nil {
 		target.Status = StatusUnhealthy
@@ -402,7 +402,7 @@ func (m *Monitor) performHealthCheck(target *Target) {
 	}
 
 	// Perform request
-	resp, err := m.client.Do(req)
+	resp, err := client.Do(req)
 	if err != nil {
 		target.Status = StatusUnhealthy
 		target.LastError = fmt.Sprintf("request failed: %v", err)