feat(encoding): encode usize and isize

Implement `EncodeGaugeValue` for `usize` and `isize`, and
`EncodeCounterValue` for `usize`. This is very straight forward and the
argument is the same as the one for `EncodeGaugeValue for u64`; if you
don't do this the user code will get littered with `as i64`/`as u64`.

Since these conversions are infallible for `32-bit` platforms, and have
the same issues as the current `u64` impl for `64-bit` platforms, I
don't think there is any problem with adding them.

Signed-off-by: Jalil David Salamé Messina <[email protected]>

Author: jalil-salame

CHANGELOG.md

@@ -13,7 +13,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `EncodeGaugeValue` would not error when encoding some `u64`s that don't fit
   in a `i64`. See [PR 281].
 
+- `EncodeGaugeValue` is implemented for `usize` and `isize`, and
+  `EncodeCounterValue` is implemented for `usize`. See [PR 282].
+
 [PR 281]: https://github.com/prometheus/client_rust/pull/281
+[PR 282]: https://github.com/prometheus/client_rust/pull/282
 
 ## [0.24.0]
 

src/encoding.rs

@@ -607,6 +607,21 @@ impl EncodeGaugeValue for u64 {
     }
 }
 
+impl EncodeGaugeValue for isize {
+    fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
+        // Is infallible for 32-bit or 64-bit platforms
+        encoder.encode_i64(i64::try_from(*self).map_err(|_err| std::fmt::Error)?)
+    }
+}
+
+impl EncodeGaugeValue for usize {
+    fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
+        // For 32-bit platforms this is infallible, for 64-bit platforms the argument is the same
+        // as the one for u64 values
+        encoder.encode_i64(i64::try_from(*self).map_err(|_err| std::fmt::Error)?)
+    }
+}
+
 impl EncodeGaugeValue for f64 {
     fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
         encoder.encode_f64(*self)
@@ -675,6 +690,13 @@ impl EncodeCounterValue for u64 {
     }
 }
 
+impl EncodeCounterValue for usize {
+    fn encode(&self, encoder: &mut CounterValueEncoder) -> Result<(), std::fmt::Error> {
+        // Is infallible for 32-bit and 64-bit platforms
+        encoder.encode_u64(u64::try_from(*self).map_err(|_err| std::fmt::Error)?)
+    }
+}
+
 impl EncodeCounterValue for f64 {
     fn encode(&self, encoder: &mut CounterValueEncoder) -> Result<(), std::fmt::Error> {
         encoder.encode_f64(*self)