[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

management/server/activity/codes.go

@@ -276,6 +276,11 @@ const (
 	// AgentNetworkSettingsUpdated indicates that a user updated Agent Network account settings
 	AgentNetworkSettingsUpdated Activity = 139
 
+	// AccountMetricsPushEnabled indicates that a user enabled metrics push for the account
+	AccountMetricsPushEnabled Activity = 140
+	// AccountMetricsPushDisabled indicates that a user disabled metrics push for the account
+	AccountMetricsPushDisabled Activity = 141
+
 	AccountDeleted Activity = 99999
 )
 
@@ -449,6 +454,9 @@ var activityMap = map[Activity]Code{
 
 	AgentNetworkSettingsUpdated: {"Agent Network settings updated", "agent_network.settings.update"},
 
+	AccountMetricsPushEnabled:  {"Account metrics push enabled", "account.setting.metrics.push.enable"},
+	AccountMetricsPushDisabled: {"Account metrics push disabled", "account.setting.metrics.push.disable"},
+
 	DomainAdded:     {"Domain added", "domain.add"},
 	DomainDeleted:   {"Domain deleted", "domain.delete"},
 	DomainValidated: {"Domain validated", "domain.validate"},