refactor(metrics/histogram): constructor accepts IntoIterator

this is a very small, **non-breaking**, alteration to the signature of
`Histogram`'s constructor.

rather than accepting an `impl Iterator`, this commit changes the
parameter of `Histogram::new()` to be an `impl IntoIterator` instead.
this does not affect the existing contract because of the blanket
`impl<I: Iterator> IntoIterator for I` implementation provided by the
standard library.

by accepting `IntoIterator` however, callers providing a collection such
as a `[f64; 5]` array or a `Vec<f64>` vector do not need to invoke
`into_iter()` themselves at the call-site.

```rust
// now, constructing a histogram needn't involve `into_iter()`...
use prometheus_client::metrics::histogram::Histogram;
let histogram = Histogram::new([10.0, 100.0, 1_000.0]);
```

this leans on the same sugar used by `for {}` loops, see the relevant
section of the `std::iter` documentation here:
<https://doc.rust-lang.org/stable/std/iter/index.html#for-loops-and-intoiterator>

no changes are needed within `Histogram::new()` because we already call
`into_iter()` on the provider iterator when appending `f64::MAX` and
collecting the buckets into a `Vec<_>`.

Signed-off-by: katelyn martin <[email protected]>

Author: cratelyn

src/metrics/histogram.rs

@@ -28,7 +28,7 @@ use std::sync::Arc;
 /// let custom_buckets = [
 ///    0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10.0,
 /// ];
-/// let histogram = Histogram::new(custom_buckets.into_iter());
+/// let histogram = Histogram::new(custom_buckets);
 /// histogram.observe(4.2);
 /// ```
 // TODO: Consider using atomics. See
@@ -57,7 +57,12 @@ pub(crate) struct Inner {
 
 impl Histogram {
     /// Create a new [`Histogram`].
-    pub fn new(buckets: impl Iterator<Item = f64>) -> Self {
+    ///
+    /// ```rust
+    /// # use prometheus_client::metrics::histogram::Histogram;
+    /// let histogram = Histogram::new([10.0, 100.0, 1_000.0]);
+    /// ```
+    pub fn new(buckets: impl IntoIterator<Item = f64>) -> Self {
         Self {
             inner: Arc::new(RwLock::new(Inner {
                 sum: Default::default(),