feat(sdk): add DO_NOT_TRACK and SHM_COLLECT_SYSTEM_METRICS env vars

- DO_NOT_TRACK=true|1 completely disables telemetry (overrides config)
- SHM_COLLECT_SYSTEM_METRICS=false|0 disables system metrics collection
- Applied to both Go and Node.js SDKs
- Updated documentation (README, SDK READMEs, DEPLOYMENT.md)

Author: btouchard

README.md

@@ -344,6 +344,24 @@ graph LR
 *   **Authentication:** The server uses a "Trust on First Use" (TOFU) or explicit activation model. Once an ID is registered with a Public Key, only that key can sign updates.
 *   **Transparency:** You should always inform your users that telemetry is active and allow them to opt-out via the `Enabled: false` config.
 
+### Respecting User Privacy with `DO_NOT_TRACK`
+
+All SHM clients (Go, Node.js, and future SDKs) respect the standard `DO_NOT_TRACK` environment variable. When set to `true` or `1`, **all telemetry is completely disabled** — no data is sent to the server.
+
+```bash
+# Disable all telemetry
+export DO_NOT_TRACK=true
+```
+
+This allows end-users to opt-out of telemetry at the system level, regardless of the application's configuration.
+
+### Environment Variables (Client-side)
+
+| Variable | Effect |
+|----------|--------|
+| `DO_NOT_TRACK=true` or `1` | Completely disables telemetry (no network requests) |
+| `SHM_COLLECT_SYSTEM_METRICS=false` or `0` | Disables automatic system metrics collection (OS, CPU, memory) while still sending custom metrics |
+
 ---
 
 ## 🤝 Contributing

docs/DEPLOYMENT.md

@@ -400,6 +400,8 @@ caddy hash-password --plaintext 'your-secure-password'
 
 ## Environment Variables
 
+### Server-side
+
 | Variable | Default | Description |
 |----------|---------|-------------|
 | `SHM_DB_DSN` | (required) | PostgreSQL connection string |
@@ -408,6 +410,34 @@ caddy hash-password --plaintext 'your-secure-password'
 
 For the full list of environment variables (including rate limiting), see [README.md](../README.md#environment-variables).
 
+### Client-side (SDK)
+
+These environment variables are read by the SHM clients (Go, Node.js) running in your applications:
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `DO_NOT_TRACK` | - | Set to `true` or `1` to **completely disable telemetry**. No data will be sent to the server. This overrides the `enabled` configuration option. |
+| `SHM_COLLECT_SYSTEM_METRICS` | `true` | Set to `false` or `0` to disable automatic system metrics collection (OS, CPU, memory). Custom metrics will still be sent. |
+
+#### Respecting User Privacy
+
+All SHM clients respect the standard `DO_NOT_TRACK` environment variable. This allows end-users to opt-out of telemetry at the system level:
+
+```bash
+# In the environment where your application runs
+export DO_NOT_TRACK=true
+```
+
+When `DO_NOT_TRACK` is enabled:
+- No network requests are made to the SHM server
+- No identity file is accessed
+- The client silently disables itself
+
+This is useful for:
+- Users who want to opt-out of all telemetry
+- Development/testing environments
+- Privacy-conscious deployments
+
 ---
 
 ## Database Backups

sdk/golang/README.md

@@ -65,6 +65,28 @@ func main() {
 | `Environment` | `string` | `""` | Environment identifier (production, staging, etc.) |
 | `Enabled` | `bool` | `false` | Enable/disable telemetry |
 | `ReportInterval` | `time.Duration` | `1h` | Interval between snapshots (minimum: 1m) |
+| `CollectSystemMetrics` | `bool` | `false` | Collect OS/runtime metrics (use `CollectSystemMetricsFromEnv()`) |
+
+## Environment Variables
+
+| Variable | Effect |
+|----------|--------|
+| `DO_NOT_TRACK=true` or `1` | **Completely disables telemetry** — overrides `Enabled: true` |
+| `SHM_COLLECT_SYSTEM_METRICS=false` or `0` | Disables system metrics collection (enabled by default) |
+
+### Example with environment variables
+
+```go
+client, _ := shm.New(shm.Config{
+    ServerURL:            "https://telemetry.example.com",
+    AppName:              "my-app",
+    AppVersion:           "1.0.0",
+    Enabled:              true,
+    CollectSystemMetrics: shm.CollectSystemMetricsFromEnv(), // reads SHM_COLLECT_SYSTEM_METRICS
+})
+```
+
+> **Note:** If `DO_NOT_TRACK=true` or `DO_NOT_TRACK=1` is set, the client will be disabled regardless of the `Enabled` configuration.
 
 ## How It Works
 

sdk/golang/client.go

@@ -20,13 +20,14 @@ import (
 )
 
 type Config struct {
-	ServerURL      string
-	AppName        string
-	AppVersion     string
-	DataDir        string // where is store app_shm_identity.json
-	Environment    string // prod, staging, ...
-	Enabled        bool
-	ReportInterval time.Duration // snapshots interval (default: 1h)
+	ServerURL            string
+	AppName              string
+	AppVersion           string
+	DataDir              string        // where is store app_shm_identity.json
+	Environment          string        // prod, staging, ...
+	Enabled              bool
+	ReportInterval       time.Duration // snapshots interval (default: 1h)
+	CollectSystemMetrics bool          // collect OS/runtime metrics (env: SHM_COLLECT_SYSTEM_METRICS)
 }
 
 type MetricsProvider func() map[string]interface{}
@@ -50,6 +51,10 @@ func New(cfg Config) (*Client, error) {
 		cfg.ReportInterval = time.Minute
 	}
 
+	if isDoNotTrack() {
+		cfg.Enabled = false
+	}
+
 	ensureDataDir(cfg.DataDir)
 	idPath := cfg.DataDir + "/" + slug(cfg.AppName) + "_shm_identity.json"
 	id, err := loadOrGenerateIdentity(idPath)
@@ -157,9 +162,11 @@ func (c *Client) sendSnapshot() {
 	if c.provider != nil {
 		data = c.provider()
 	}
-	sysData := c.getSystemMetrics()
-	for k, v := range sysData {
-		data[k] = v
+	if c.config.CollectSystemMetrics {
+		sysData := c.getSystemMetrics()
+		for k, v := range sysData {
+			data[k] = v
+		}
 	}
 	metricsJSON, _ := json.Marshal(data)
 
@@ -253,6 +260,16 @@ func ensureDataDir(dir string) {
 	}
 }
 
+func CollectSystemMetricsFromEnv() bool {
+	val := strings.ToLower(os.Getenv("SHM_COLLECT_SYSTEM_METRICS"))
+	return val != "false" && val != "0"
+}
+
+func isDoNotTrack() bool {
+	val := strings.ToLower(os.Getenv("DO_NOT_TRACK"))
+	return val == "true" || val == "1"
+}
+
 func slug(s string) string {
 	s = strings.ToLower(s)
 

sdk/golang/client_test.go

@@ -600,3 +600,191 @@ func TestClient_RegisterError_BadStatusCode(t *testing.T) {
 		t.Errorf("error should mention status code: %v", err)
 	}
 }
+
+// =============================================================================
+// COLLECT SYSTEM METRICS TESTS
+// =============================================================================
+
+func TestClient_SnapshotWithSystemMetrics(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	var receivedMetrics map[string]interface{}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/v1/snapshot" {
+			bodyBytes, _ := io.ReadAll(r.Body)
+			var body map[string]interface{}
+			json.Unmarshal(bodyBytes, &body)
+			if metricsRaw, ok := body["metrics"].(string); ok {
+				json.Unmarshal([]byte(metricsRaw), &receivedMetrics)
+			} else if metricsMap, ok := body["metrics"].(map[string]interface{}); ok {
+				receivedMetrics = metricsMap
+			}
+		}
+		w.WriteHeader(http.StatusAccepted)
+	}))
+	defer server.Close()
+
+	cfg := Config{
+		ServerURL:            server.URL,
+		AppName:              "test-app",
+		AppVersion:           "1.0.0",
+		DataDir:              tmpDir,
+		Enabled:              true,
+		CollectSystemMetrics: true,
+	}
+
+	client, _ := New(cfg)
+	client.SetProvider(func() map[string]interface{} {
+		return map[string]interface{}{"custom_metric": 123}
+	})
+	client.sendSnapshot()
+
+	// Should contain system metrics
+	systemFields := []string{"sys_os", "sys_arch", "sys_cpu_cores", "sys_go_version", "sys_mode"}
+	for _, field := range systemFields {
+		if _, ok := receivedMetrics[field]; !ok {
+			t.Errorf("expected system metric %q when CollectSystemMetrics=true", field)
+		}
+	}
+
+	// Should also contain custom metric
+	if receivedMetrics["custom_metric"] != float64(123) {
+		t.Errorf("custom_metric = %v, want 123", receivedMetrics["custom_metric"])
+	}
+}
+
+func TestClient_SnapshotWithoutSystemMetrics(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	var receivedMetrics map[string]interface{}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/v1/snapshot" {
+			bodyBytes, _ := io.ReadAll(r.Body)
+			var body map[string]interface{}
+			json.Unmarshal(bodyBytes, &body)
+			if metricsRaw, ok := body["metrics"].(string); ok {
+				json.Unmarshal([]byte(metricsRaw), &receivedMetrics)
+			} else if metricsMap, ok := body["metrics"].(map[string]interface{}); ok {
+				receivedMetrics = metricsMap
+			}
+		}
+		w.WriteHeader(http.StatusAccepted)
+	}))
+	defer server.Close()
+
+	cfg := Config{
+		ServerURL:            server.URL,
+		AppName:              "test-app",
+		AppVersion:           "1.0.0",
+		DataDir:              tmpDir,
+		Enabled:              true,
+		CollectSystemMetrics: false,
+	}
+
+	client, _ := New(cfg)
+	client.SetProvider(func() map[string]interface{} {
+		return map[string]interface{}{"custom_metric": 456}
+	})
+	client.sendSnapshot()
+
+	// Should NOT contain system metrics
+	systemFields := []string{"sys_os", "sys_arch", "sys_cpu_cores", "sys_go_version", "sys_mode"}
+	for _, field := range systemFields {
+		if _, ok := receivedMetrics[field]; ok {
+			t.Errorf("unexpected system metric %q when CollectSystemMetrics=false", field)
+		}
+	}
+
+	// Should still contain custom metric
+	if receivedMetrics["custom_metric"] != float64(456) {
+		t.Errorf("custom_metric = %v, want 456", receivedMetrics["custom_metric"])
+	}
+}
+
+func TestCollectSystemMetricsFromEnv(t *testing.T) {
+	tests := []struct {
+		envValue string
+		expected bool
+	}{
+		{"", true},      // absent = enabled
+		{"true", true},
+		{"TRUE", true},
+		{"1", true},
+		{"false", false},
+		{"FALSE", false},
+		{"0", false},
+		{"anything", true}, // unknown = enabled
+	}
+
+	for _, tt := range tests {
+		t.Run("env="+tt.envValue, func(t *testing.T) {
+			if tt.envValue == "" {
+				os.Unsetenv("SHM_COLLECT_SYSTEM_METRICS")
+			} else {
+				os.Setenv("SHM_COLLECT_SYSTEM_METRICS", tt.envValue)
+			}
+			defer os.Unsetenv("SHM_COLLECT_SYSTEM_METRICS")
+
+			result := CollectSystemMetricsFromEnv()
+			if result != tt.expected {
+				t.Errorf("CollectSystemMetricsFromEnv() with env=%q = %v, want %v", tt.envValue, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestDoNotTrack(t *testing.T) {
+	tests := []struct {
+		envValue string
+		expected bool
+	}{
+		{"", false},        // absent = tracking allowed
+		{"true", true},     // disabled
+		{"TRUE", true},     // disabled
+		{"1", true},        // disabled
+		{"false", false},   // tracking allowed
+		{"0", false},       // tracking allowed
+		{"anything", false}, // unknown = tracking allowed
+	}
+
+	for _, tt := range tests {
+		t.Run("DO_NOT_TRACK="+tt.envValue, func(t *testing.T) {
+			if tt.envValue == "" {
+				os.Unsetenv("DO_NOT_TRACK")
+			} else {
+				os.Setenv("DO_NOT_TRACK", tt.envValue)
+			}
+			defer os.Unsetenv("DO_NOT_TRACK")
+
+			result := isDoNotTrack()
+			if result != tt.expected {
+				t.Errorf("isDoNotTrack() with env=%q = %v, want %v", tt.envValue, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestClient_DoNotTrack_DisablesClient(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	os.Setenv("DO_NOT_TRACK", "true")
+	defer os.Unsetenv("DO_NOT_TRACK")
+
+	cfg := Config{
+		ServerURL:  "http://localhost:8080",
+		AppName:    "test-app",
+		AppVersion: "1.0.0",
+		DataDir:    tmpDir,
+		Enabled:    true, // explicitly enabled
+	}
+
+	client, err := New(cfg)
+	if err != nil {
+		t.Fatalf("New() error = %v", err)
+	}
+
+	// DO_NOT_TRACK should override Enabled
+	if client.config.Enabled != false {
+		t.Errorf("config.Enabled = %v, want false when DO_NOT_TRACK=true", client.config.Enabled)
+	}
+}

sdk/nodejs/README.md

@@ -50,9 +50,33 @@ interface Config {
   environment?: string;     // Environment identifier (production, staging, etc.)
   enabled?: boolean;        // Enable/disable telemetry (default: true)
   reportIntervalMs?: number; // Interval between snapshots in ms (default: 3600000, min: 60000)
+  collectSystemMetrics?: boolean; // Collect OS/runtime metrics (default: true via env)
 }
 ```
 
+## Environment Variables
+
+| Variable | Effect |
+|----------|--------|
+| `DO_NOT_TRACK=true` or `1` | **Completely disables telemetry** — overrides `enabled: true` |
+| `SHM_COLLECT_SYSTEM_METRICS=false` or `0` | Disables system metrics collection (enabled by default) |
+
+### Example with environment variables
+
+```typescript
+import { SHMClient, collectSystemMetricsFromEnv } from '@btouchard/shm-sdk';
+
+const client = new SHMClient({
+  serverUrl: 'https://telemetry.example.com',
+  appName: 'my-app',
+  appVersion: '1.0.0',
+  enabled: true,
+  collectSystemMetrics: collectSystemMetricsFromEnv(), // reads SHM_COLLECT_SYSTEM_METRICS
+});
+```
+
+> **Note:** If `DO_NOT_TRACK=true` or `DO_NOT_TRACK=1` is set, the client will be disabled regardless of the `enabled` configuration.
+
 ## How It Works
 
 1. **Identity Generation**: On first run, the SDK generates an Ed25519 keypair and a unique instance ID, stored in `{dataDir}/{app-name}_shm_identity.json`