feat(pageserver): use hostname as feature flag resolver property (#12141)

## Problem

part of https://github.com/neondatabase/neon/issues/11813

## Summary of changes

Collect pageserver hostname property so that we can use it in the
PostHog UI. Not sure if this is the best way to do that -- open to
suggestions.

---------

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

libs/remote_storage/src/s3_bucket.rs

@@ -1022,6 +1022,7 @@ impl RemoteStorage for S3Bucket {
             let Version { key, .. } = &vd;
             let version_id = vd.version_id().map(|v| v.0.as_str());
             if version_id == Some("null") {
+                // TODO: check the behavior of using the SDK on a non-versioned container
                 return Err(TimeTravelError::Other(anyhow!(
                     "Received ListVersions response for key={key} with version_id='null', \
                     indicating either disabled versioning, or legacy objects with null version id values"

pageserver/src/feature_resolver.rs

@@ -1,5 +1,6 @@
 use std::{collections::HashMap, sync::Arc, time::Duration};
 
+use pageserver_api::config::NodeMetadata;
 use posthog_client_lite::{
     CaptureEvent, FeatureResolverBackgroundLoop, PostHogClientConfig, PostHogEvaluationError,
     PostHogFlagFilterPropertyValue,
@@ -86,7 +87,35 @@ impl FeatureResolver {
                         }
                     }
                 }
-                // TODO: add pageserver URL.
+                // TODO: move this to a background task so that we don't block startup in case of slow disk
+                let metadata_path = conf.metadata_path();
+                match std::fs::read_to_string(&metadata_path) {
+                    Ok(metadata_str) => match serde_json::from_str::<NodeMetadata>(&metadata_str) {
+                        Ok(metadata) => {
+                            properties.insert(
+                                "hostname".to_string(),
+                                PostHogFlagFilterPropertyValue::String(metadata.http_host),
+                            );
+                            if let Some(cplane_region) = metadata.other.get("region_id") {
+                                if let Some(cplane_region) = cplane_region.as_str() {
+                                    // This region contains the cell number
+                                    properties.insert(
+                                        "neon_region".to_string(),
+                                        PostHogFlagFilterPropertyValue::String(
+                                            cplane_region.to_string(),
+                                        ),
+                                    );
+                                }
+                            }
+                        }
+                        Err(e) => {
+                            tracing::warn!("Failed to parse metadata.json: {}", e);
+                        }
+                    },
+                    Err(e) => {
+                        tracing::warn!("Failed to read metadata.json: {}", e);
+                    }
+                }
                 Arc::new(properties)
             };
             let fake_tenants = {