feat(pageserver): observability for feature flags (#12034)

## Problem

Part of #11813. This pull request adds misc observability improvements
for the functionality.

## Summary of changes

* Info span for the PostHog feature background loop.
* New evaluate feature flag API.
* Put the request error into the error message.
* Log when feature flag gets updated.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/posthog_client_lite/src/background_loop.rs

@@ -4,6 +4,7 @@ use std::{sync::Arc, time::Duration};
 
 use arc_swap::ArcSwap;
 use tokio_util::sync::CancellationToken;
+use tracing::{Instrument, info_span};
 
 use crate::{FeatureStore, PostHogClient, PostHogClientConfig};
 
@@ -26,31 +27,35 @@ impl FeatureResolverBackgroundLoop {
     pub fn spawn(self: Arc<Self>, handle: &tokio::runtime::Handle, refresh_period: Duration) {
         let this = self.clone();
         let cancel = self.cancel.clone();
-        handle.spawn(async move {
-            tracing::info!("Starting PostHog feature resolver");
-            let mut ticker = tokio::time::interval(refresh_period);
-            ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
-            loop {
-                tokio::select! {
-                    _ = ticker.tick() => {}
-                    _ = cancel.cancelled() => break
-                }
-                let resp = match this
-                    .posthog_client
-                    .get_feature_flags_local_evaluation()
-                    .await
-                {
-                    Ok(resp) => resp,
-                    Err(e) => {
-                        tracing::warn!("Cannot get feature flags: {}", e);
-                        continue;
+        handle.spawn(
+            async move {
+                tracing::info!("Starting PostHog feature resolver");
+                let mut ticker = tokio::time::interval(refresh_period);
+                ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
+                loop {
+                    tokio::select! {
+                        _ = ticker.tick() => {}
+                        _ = cancel.cancelled() => break
                     }
-                };
-                let feature_store = FeatureStore::new_with_flags(resp.flags);
-                this.feature_store.store(Arc::new(feature_store));
+                    let resp = match this
+                        .posthog_client
+                        .get_feature_flags_local_evaluation()
+                        .await
+                    {
+                        Ok(resp) => resp,
+                        Err(e) => {
+                            tracing::warn!("Cannot get feature flags: {}", e);
+                            continue;
+                        }
+                    };
+                    let feature_store = FeatureStore::new_with_flags(resp.flags);
+                    this.feature_store.store(Arc::new(feature_store));
+                    tracing::info!("Feature flag updated");
+                }
+                tracing::info!("PostHog feature resolver stopped");
             }
-            tracing::info!("PostHog feature resolver stopped");
-        });
+            .instrument(info_span!("posthog_feature_resolver")),
+        );
     }
 
     pub fn feature_store(&self) -> Arc<FeatureStore> {

libs/posthog_client_lite/src/lib.rs

@@ -448,6 +448,18 @@ impl FeatureStore {
             )))
         }
     }
+
+    /// Infer whether a feature flag is a boolean flag by checking if it has a multivariate filter.
+    pub fn is_feature_flag_boolean(&self, flag_key: &str) -> Result<bool, PostHogEvaluationError> {
+        if let Some(flag_config) = self.flags.get(flag_key) {
+            Ok(flag_config.filters.multivariate.is_none())
+        } else {
+            Err(PostHogEvaluationError::NotAvailable(format!(
+                "Not found in the local evaluation spec: {}",
+                flag_key
+            )))
+        }
+    }
 }
 
 pub struct PostHogClientConfig {
@@ -528,7 +540,15 @@ impl PostHogClient {
             .bearer_auth(&self.config.server_api_key)
             .send()
             .await?;
+        let status = response.status();
         let body = response.text().await?;
+        if !status.is_success() {
+            return Err(anyhow::anyhow!(
+                "Failed to get feature flags: {}, {}",
+                status,
+                body
+            ));
+        }
         Ok(serde_json::from_str(&body)?)
     }