fix(telemetry): correctly deserialise feature flags (#8425)

Our Posthog integration was so lenient in regards to errors that I
didn't even notice at all that we failed to deserialise them correctly.
In Posthog, I configured the feature flags with `kebab-case` but we
tried to deserialise them as `snake_case`.

rust/telemetry/src/lib.rs

@@ -5,7 +5,7 @@ use std::{
     time::Duration,
 };
 
-use anyhow::{Context, Result};
+use anyhow::{bail, Context, Result};
 use env::ON_PREM;
 use parking_lot::RwLock;
 use sentry::protocol::SessionStatus;
@@ -210,30 +210,47 @@ fn set_current_user(user: Option<sentry::User>) {
     sentry::Hub::main().configure_scope(|scope| scope.set_user(user));
 }
 
-fn evaluate_feature_flags(id: String) -> Result<FeatureFlags> {
-    let json = reqwest::blocking::Client::new()
+fn evaluate_feature_flags(distinct_id: String) -> Result<FeatureFlags> {
+    let response = reqwest::blocking::ClientBuilder::new()
+        .connection_verbose(true)
+        .build()?
         .post("https://us.i.posthog.com/decide?v=3")
-        .body(format!(
-            r#"{{
-            "api_key": "{POSTHOG_API_KEY}",
-            "distinct_id": "{id}"
-        }}"#
-        ))
+        .json(&DecideRequest {
+            api_key: POSTHOG_API_KEY.to_string(),
+            distinct_id,
+        })
         .send()
-        .context("Failed to send POST request")?
+        .context("Failed to send POST request")?;
+
+    let status = response.status();
+
+    if !status.is_success() {
+        let body = response.text().unwrap_or_default();
+
+        bail!("Failed to get feature flags; status={status}, body={body}")
+    }
+
+    let json = response
         .json::<DecideResponse>()
         .context("Failed to deserialize response")?;
 
     Ok(json.feature_flags)
 }
 
-#[derive(Debug, Serialize, Deserialize)]
+#[derive(Debug, Serialize)]
+struct DecideRequest {
+    api_key: String,
+    distinct_id: String,
+}
+
+#[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 struct DecideResponse {
     feature_flags: FeatureFlags,
 }
 
-#[derive(Debug, Serialize, Deserialize, Default, Clone, Copy)]
+#[derive(Debug, Deserialize, Default, Clone, Copy)]
+#[serde(rename_all = "kebab-case")]
 struct FeatureFlags {
     #[serde(default)]
     icmp_unreachable_instead_of_nat64: bool,