pageserver/storcon: add patch endpoints for tenant config metrics (#10020)

## Problem

Cplane and storage controller tenant config changes are not additive.
Any change overrides all existing tenant configs. This would be fine if
both did client side patching, but that's not the case.

Once this merges, we must update cplane to use the PATCH endpoint.

## Summary of changes

### High Level

Allow for patching of tenant configuration with a `PATCH
/v1/tenant/config` endpoint.
It takes the same data as it's PUT counterpart. For example the payload
below will update `gc_period` and unset `compaction_period`. All other
fields are left in their original state.
```
{
  "tenant_id": "1234",
  "gc_period": "10s",
  "compaction_period": null
}
```

### Low Level
* PS and storcon gain `PATCH /v1/tenant/config` endpoints. PS endpoint
is only used for cplane managed instances.
* `storcon_cli` is updated to have separate commands for
`set-tenant-config` and `patch-tenant-config`

Related https://github.com/neondatabase/cloud/issues/21043

libs/utils/src/lib.rs

@@ -94,6 +94,8 @@ pub mod toml_edit_ext;
 
 pub mod circuit_breaker;
 
+pub mod try_rcu;
+
 // Re-export used in macro. Avoids adding git-version as dep in target crates.
 #[doc(hidden)]
 pub use git_version;

libs/utils/src/try_rcu.rs

@@ -0,0 +1,77 @@
+//! Try RCU extension lifted from <https://github.com/vorner/arc-swap/issues/94#issuecomment-1987154023>
+
+pub trait ArcSwapExt<T> {
+    /// [`ArcSwap::rcu`](arc_swap::ArcSwap::rcu), but with Result that short-circuits on error.
+    fn try_rcu<R, F, E>(&self, f: F) -> Result<T, E>
+    where
+        F: FnMut(&T) -> Result<R, E>,
+        R: Into<T>;
+}
+
+impl<T, S> ArcSwapExt<T> for arc_swap::ArcSwapAny<T, S>
+where
+    T: arc_swap::RefCnt,
+    S: arc_swap::strategy::CaS<T>,
+{
+    fn try_rcu<R, F, E>(&self, mut f: F) -> Result<T, E>
+    where
+        F: FnMut(&T) -> Result<R, E>,
+        R: Into<T>,
+    {
+        fn ptr_eq<Base, A, B>(a: A, b: B) -> bool
+        where
+            A: arc_swap::AsRaw<Base>,
+            B: arc_swap::AsRaw<Base>,
+        {
+            let a = a.as_raw();
+            let b = b.as_raw();
+            std::ptr::eq(a, b)
+        }
+
+        let mut cur = self.load();
+        loop {
+            let new = f(&cur)?.into();
+            let prev = self.compare_and_swap(&*cur, new);
+            let swapped = ptr_eq(&*cur, &*prev);
+            if swapped {
+                return Ok(arc_swap::Guard::into_inner(prev));
+            } else {
+                cur = prev;
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use arc_swap::ArcSwap;
+    use std::sync::Arc;
+
+    #[test]
+    fn test_try_rcu_success() {
+        let swap = ArcSwap::from(Arc::new(42));
+
+        let result = swap.try_rcu(|value| -> Result<_, String> { Ok(**value + 1) });
+
+        assert!(result.is_ok());
+        assert_eq!(**swap.load(), 43);
+    }
+
+    #[test]
+    fn test_try_rcu_error() {
+        let swap = ArcSwap::from(Arc::new(42));
+
+        let result = swap.try_rcu(|value| -> Result<i32, _> {
+            if **value == 42 {
+                Err("err")
+            } else {
+                Ok(**value + 1)
+            }
+        });
+
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err(), "err");
+        assert_eq!(**swap.load(), 42);
+    }
+}