Fix value toml parsing and add a ut

pageserver/src/config.rs

@@ -1330,6 +1330,19 @@ trace_read_requests = {trace_read_requests}"#,
         Ok(())
     }
 
+    #[test]
+    fn parse_override_tenant_config() -> anyhow::Result<()> {
+        let config_string = r#"tenant_config={ min_resident_size_override =  400 }"#.to_string();
+
+        let toml: Document = config_string.parse()?;
+        let item = toml.get("tenant_config").unwrap();
+        let conf = TenantConfOpt::try_from(item.to_owned()).unwrap();
+
+        assert_eq!(conf.min_resident_size_override, Some(400));
+
+        Ok(())
+    }
+
     #[test]
     fn eviction_pageserver_config_parse() -> anyhow::Result<()> {
         let tempdir = tempdir()?;

pageserver/src/tenant/config.rs

@@ -546,11 +546,9 @@ impl TryFrom<toml_edit::Item> for TenantConfOpt {
     fn try_from(item: toml_edit::Item) -> Result<Self, Self::Error> {
         match item {
             toml_edit::Item::Value(value) => {
-                let mut table = toml_edit::Table::new();
-                table["value"] = toml_edit::Item::Value(value);
-                let deserializer = toml_edit::de::Deserializer::new(table.into());
-                serde_path_to_error::deserialize(deserializer)
-                    .map_err(|e| anyhow::anyhow!("{}: {}", e.path(), e.inner().message()))
+                let d = value.into_deserializer();
+                return serde_path_to_error::deserialize(d)
+                    .map_err(|e| anyhow::anyhow!("{}: {}", e.path(), e.inner().message()));
             }
             toml_edit::Item::Table(table) => {
                 let deserializer = toml_edit::de::Deserializer::new(table.into());