[management, client] Add management-controlled client metrics push

Allow enabling/disabling client metrics push from the dashboard via
account settings instead of requiring env vars on every client.

- Add MetricsConfig proto message to NetbirdConfig
- Add MetricsPushEnabled to account Settings (DB-persisted)
- Expose metrics_push_enabled in OpenAPI and dashboard API handler
- Populate MetricsConfig in sync and login responses
- Client dynamically starts/stops push based on management config
- NB_METRICS_PUSH_ENABLED env var overrides management when explicitly set
- Add activity events for metrics push enable/disable

shared/management/proto/management.proto

@@ -312,6 +312,8 @@ message NetbirdConfig {
   RelayConfig relay = 4;
 
   FlowConfig flow = 5;
+
+  MetricsConfig metrics = 6;
 }
 
 // HostConfig describes connection properties of some server (e.g. STUN, Signal, Management)
@@ -350,6 +352,10 @@ message FlowConfig {
   bool dnsCollection = 8;
 }
 
+message MetricsConfig {
+  bool enabled = 1;
+}
+
 // JWTConfig represents JWT authentication configuration for validating tokens.
 message JWTConfig {
   string issuer = 1;

shared/management/proto/proxy_service.pb.go

@@ -1,7 +1,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.26.0
-// 	protoc        v7.34.1
+// 	protoc        v6.33.1
 // source: proxy_service.proto
 
 package proto