feat(encoding): implement EncodeLabelValue for Option (#141)

Signed-off-by: Evan Schwartz <3262610+emschwartz@users.noreply.github.com>
Co-authored-by: Evan Schwartz <3262610+emschwartz@users.noreply.github.com>

Author: mxinden

CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.21.1]
+
+### Added
+
+- Implement `EncodeLabelValue` for `Option<T>`.
+  See [PR 137].
+
+[PR 137]: https://github.com/prometheus/client_rust/pull/137
+
 ## [0.21.0]
 
 ### Changed

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "prometheus-client"
-version = "0.21.0"
+version = "0.21.1"
 authors = ["Max Inden <mail@max-inden.de>"]
 edition = "2021"
 description = "Open Metrics client library allowing users to natively instrument applications."

src/encoding.rs

@@ -404,6 +404,18 @@ impl EncodeLabelValue for f64 {
     }
 }
 
+impl<T> EncodeLabelValue for Option<T>
+where
+    T: EncodeLabelValue,
+{
+    fn encode(&self, encoder: &mut LabelValueEncoder) -> Result<(), std::fmt::Error> {
+        match self {
+            Some(v) => EncodeLabelValue::encode(v, encoder),
+            None => EncodeLabelValue::encode(&"", encoder),
+        }
+    }
+}
+
 macro_rules! impl_encode_label_value_for_integer {
     ($($t:ident),*) => {$(
         impl EncodeLabelValue for $t {