feat(app-server): add an --analytics-default-enabled flag (#9118)

Add a new `codex app-server --analytics-default-enabled` CLI flag that
controls whether analytics are enabled by default.

Analytics are disabled by default for app-server. Users have to
explicitly opt in
via the `analytics` section in the config.toml file.

However, for first-party use cases like the VSCode IDE extension, we
default analytics
to be enabled by default by setting this flag. Users can still opt out
by setting this
in their config.toml:

```toml
[analytics]
enabled = false
```

See https://developers.openai.com/codex/config-advanced/#metrics for
more details.

codex-rs/app-server/tests/suite/v2/analytics.rs

@@ -0,0 +1,66 @@
+use anyhow::Result;
+use codex_core::config::ConfigBuilder;
+use codex_core::config::types::OtelExporterKind;
+use codex_core::config::types::OtelHttpProtocol;
+use pretty_assertions::assert_eq;
+use std::collections::HashMap;
+use tempfile::TempDir;
+
+const SERVICE_VERSION: &str = "0.0.0-test";
+
+fn set_metrics_exporter(config: &mut codex_core::config::Config) {
+    config.otel.metrics_exporter = OtelExporterKind::OtlpHttp {
+        endpoint: "http://localhost:4318".to_string(),
+        headers: HashMap::new(),
+        protocol: OtelHttpProtocol::Json,
+        tls: None,
+    };
+}
+
+#[tokio::test]
+async fn app_server_default_analytics_disabled_without_flag() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .build()
+        .await?;
+    set_metrics_exporter(&mut config);
+    config.analytics_enabled = None;
+
+    let provider = codex_core::otel_init::build_provider(
+        &config,
+        SERVICE_VERSION,
+        Some("codex_app_server"),
+        false,
+    )
+    .map_err(|err| anyhow::anyhow!(err.to_string()))?;
+
+    // With analytics unset in the config and the default flag is false, metrics are disabled.
+    // No provider is built.
+    assert_eq!(provider.is_none(), true);
+    Ok(())
+}
+
+#[tokio::test]
+async fn app_server_default_analytics_enabled_with_flag() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .build()
+        .await?;
+    set_metrics_exporter(&mut config);
+    config.analytics_enabled = None;
+
+    let provider = codex_core::otel_init::build_provider(
+        &config,
+        SERVICE_VERSION,
+        Some("codex_app_server"),
+        true,
+    )
+    .map_err(|err| anyhow::anyhow!(err.to_string()))?;
+
+    // With analytics unset in the config and the default flag is true, metrics are enabled.
+    let has_metrics = provider.as_ref().and_then(|otel| otel.metrics()).is_some();
+    assert_eq!(has_metrics, true);
+    Ok(())
+}