pageserver: implement max_total_size_bytes limit for basebackup cache (#12230)

## Problem
The cache was introduced as a hackathon project and the only supported
limit was the number of entries.
The basebackup entry size may vary. We need to have more control over
disk space usage to ship it to production.

- Part of https://github.com/neondatabase/cloud/issues/29353

## Summary of changes
- Store the size of entries in the cache and use it to limit
`max_total_size_bytes`
- Add the size of the cache in bytes to metrics.

libs/pageserver_api/src/config.rs

@@ -356,16 +356,21 @@ pub struct TimelineImportConfig {
 pub struct BasebackupCacheConfig {
     #[serde(with = "humantime_serde")]
     pub cleanup_period: Duration,
-    // FIXME: Support max_size_bytes.
-    // pub max_size_bytes: usize,
-    pub max_size_entries: i64,
+    /// Maximum total size of basebackup cache entries on disk in bytes.
+    /// The cache may slightly exceed this limit because we do not know
+    /// the exact size of the cache entry untill it's written to disk.
+    pub max_total_size_bytes: u64,
+    // TODO(diko): support max_entry_size_bytes.
+    // pub max_entry_size_bytes: u64,
+    pub max_size_entries: usize,
 }
 
 impl Default for BasebackupCacheConfig {
     fn default() -> Self {
         Self {
             cleanup_period: Duration::from_secs(60),
-            // max_size_bytes: 1024 * 1024 * 1024, // 1 GiB
+            max_total_size_bytes: 1024 * 1024 * 1024, // 1 GiB
+            // max_entry_size_bytes: 16 * 1024 * 1024,   // 16 MiB
             max_size_entries: 1000,
         }
     }