libs: move metric collection for pageserver and safekeeper in a background task (#12525)

## Problem

Safekeeper and pageserver metrics collection might time out. We've seen
this in both hadron and neon.

## Summary of changes

This PR moves metrics collection in PS/SK to the background so that we
will always get some metrics, despite there may be some delays. Will
leave it to the future work to reduce metrics collection time.

---------

Co-authored-by: Chen Luo <chen.luo@databricks.com>

libs/pageserver_api/src/config.rs

@@ -274,6 +274,7 @@ pub struct ConfigToml {
     pub basebackup_cache_config: Option<BasebackupCacheConfig>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub image_layer_generation_large_timeline_threshold: Option<u64>,
+    pub force_metric_collection_on_scrape: bool,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -831,6 +832,7 @@ impl Default for ConfigToml {
             basebackup_cache_config: None,
             posthog_config: None,
             image_layer_generation_large_timeline_threshold: Some(2 * 1024 * 1024 * 1024),
+            force_metric_collection_on_scrape: true,
         }
     }
 }