Set a default "Unknown" channel

glean-core/src/core_metrics.rs

@@ -8,7 +8,7 @@ use crate::{CommonMetricData, Lifetime};
 use once_cell::sync::Lazy;
 
 /// Metrics included in every ping as `client_info`.
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct ClientInfoMetrics {
     /// The build identifier generated by the CI system (e.g. "1234/A").
     pub app_build: String,
@@ -45,7 +45,7 @@ impl ClientInfoMetrics {
             app_display_version: "Unknown".to_string(),
             architecture: "Unknown".to_string(),
             os_version: "Unknown".to_string(),
-            channel: None,
+            channel: Some("Unknown".to_string()),
             android_sdk_version: None,
             device_manufacturer: None,
             device_model: None,