Bug 1823684 - Server Knobs for iOS

changelog
- adds the new `glean-server-knobs` feature to the Nimbus config
- call `setMetricsEnabledConfig` on init & pull values directly from the Nimbus experiment

firefox-ios/Client/Telemetry/TelemetryWrapper.swift

@@ -106,6 +106,9 @@ class TelemetryWrapper: TelemetryWrapperProtocol, FeatureFlaggable {
                          configuration: gleanConfig,
                          buildInfo: GleanMetrics.GleanBuild.info)
 
+        // Set the metric configuration from Nimbus.
+        glean.setMetricsEnabledConfig(FxNimbus.shared.features.gleanServerKnobs.value().toJSONString())
+
         // Save the profile so we can record settings from it when the notification below fires.
         self.profile = profile
 

firefox-ios/nimbus-features/gleanServerKnobs.yaml

@@ -0,0 +1,8 @@
+features:
+  glean-server-knobs:
+    description: "A feature that provides server-side configurations for Glean metrics (aka Server Knobs)."
+    variables:
+      metrics-enabled:
+        description: "A map of metric base-identifiers to booleans representing the state of the 'enabled' flag for that metric."
+        type: Map<String, Boolean>
+        default: {}

firefox-ios/nimbus.fml.yaml

@@ -21,6 +21,7 @@ include:
   - nimbus-features/fakespotFeature.yaml
   - nimbus-features/feltPrivacyFeature.yaml
   - nimbus-features/firefoxSuggestFeature.yaml
+  - nimbus-features/gleanServerKnobs.yaml
   - nimbus-features/generalFeatures.yaml
   - nimbus-features/homescreenFeature.yaml
   - nimbus-features/messagingFeature.yaml