fix(pageserver): reduce default feature flag refresh interval (#12246)

## Problem

Part of #11813 

## Summary of changes

The current interval is 30s and it costs a lot of $$$. This patch
reduced it to 600s refresh interval (which means that it takes 10min for
feature flags to propagate from UI to the pageserver). In the future we
can let storcon retrieve the feature flags and push it to pageservers.
We can consider creating a new release or we can postpone this to the
week after the next week.

---------

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

libs/pageserver_api/src/config.rs

@@ -76,6 +76,10 @@ pub struct PostHogConfig {
     pub private_api_url: String,
     /// Public API URL
     pub public_api_url: String,
+    /// Refresh interval for the feature flag spec
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(with = "humantime_serde")]
+    pub refresh_interval: Option<Duration>,
 }
 
 /// `pageserver.toml`

libs/posthog_client_lite/src/background_loop.rs

@@ -36,7 +36,10 @@ impl FeatureResolverBackgroundLoop {
         // Main loop of updating the feature flags.
         handle.spawn(
             async move {
-                tracing::info!("Starting PostHog feature resolver");
+                tracing::info!(
+                    "Starting PostHog feature resolver with refresh period: {:?}",
+                    refresh_period
+                );
                 let mut ticker = tokio::time::interval(refresh_period);
                 ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
                 loop {

pageserver/src/feature_resolver.rs

@@ -12,6 +12,8 @@ use utils::id::TenantId;
 
 use crate::{config::PageServerConf, metrics::FEATURE_FLAG_EVALUATION};
 
+const DEFAULT_POSTHOG_REFRESH_INTERVAL: Duration = Duration::from_secs(600);
+
 #[derive(Clone)]
 pub struct FeatureResolver {
     inner: Option<Arc<FeatureResolverBackgroundLoop>>,
@@ -139,10 +141,13 @@ impl FeatureResolver {
                 }
                 tenants
             };
-            // TODO: make refresh period configurable
-            inner
-                .clone()
-                .spawn(handle, Duration::from_secs(60), fake_tenants);
+            inner.clone().spawn(
+                handle,
+                posthog_config
+                    .refresh_interval
+                    .unwrap_or(DEFAULT_POSTHOG_REFRESH_INTERVAL),
+                fake_tenants,
+            );
             Ok(FeatureResolver {
                 inner: Some(inner),
                 internal_properties: Some(internal_properties),