fix(pulling_gauge): reduce indirection

Previously we stored an `Arc<Box<dyn Fn()>>`, but we can directly store
the function in the `Arc`.

We could also take an `impl Fn() -> f64 + Send + Sync` instead of a
`Box<dyn Fn() -> f64 + Send + Sync>` and do the allocation inside `new`:

```rust
impl PullingGauge {
    pub fn new<S1: Into<String>, S2: Into<String>>(
        name: S1,
        help: S2,
        value: impl Fn() -> f64 + Send + Sync,
    ) -> crate::Result<Self> {
        Ok(PullingGauge {
            value: Arc::new(value),
            desc: crate::core::Desc::new(name.into(), help.into(), Vec::new(), HashMap::new())?,
        })
    }
}
```

This is more efficient and lets you write the closure directly on the
calling site:

```rust
// Previously
let pulling_gauge = PullingGauge::new("foo", "bar", Box::new(|| static_vec.len() as f64));
// Now
let pulling_gauge = PullingGauge::new("foo", "bar", || static_vec.len() as f64);
```

This wouldn't be a breaking change since `Box<dyn Fn()>` implements `Fn()`,
but it would make it less efficient since you are now storing a `Box`
inside the `Arc`.

Author: jalil-salame

src/pulling_gauge.rs

@@ -7,7 +7,7 @@ use crate::{
 
 /// A [Gauge] that returns the value from a provided function on every collect run.
 ///
-/// This metric is the equivalant of Go's
+/// This metric is the equivalent of Go's
 /// <https://pkg.go.dev/github.com/prometheus/[email protected]/prometheus#GaugeFunc>
 ///
 /// # Examples
@@ -28,7 +28,7 @@ use crate::{
 #[derive(Clone)]
 pub struct PullingGauge {
     desc: crate::core::Desc,
-    value: Arc<Box<dyn Fn() -> f64 + Send + Sync>>,
+    value: Arc<dyn Fn() -> f64 + Send + Sync>,
 }
 
 impl fmt::Debug for PullingGauge {
@@ -48,7 +48,7 @@ impl PullingGauge {
         value: Box<dyn Fn() -> f64 + Send + Sync>,
     ) -> crate::Result<Self> {
         Ok(PullingGauge {
-            value: Arc::new(value),
+            value: Arc::from(value),
             desc: crate::core::Desc::new(name.into(), help.into(), Vec::new(), HashMap::new())?,
         })
     }