feat: support multi-value key propagation extraction

opentelemetry-http/src/lib.rs

@@ -43,6 +43,16 @@
             .map(|value| value.as_str())
             .collect::<Vec<_>>()
     }
+
+    /// Get all the values for a key from the HeaderMap
+    fn get_all(&self, key: &str) -> Option<Vec<&str>> {
+        let all_iter = self.0.get_all(key).iter();
+        if let (0, Some(0)) = all_iter.size_hint() {
+            return None;
+        }
+
+        Some(all_iter.filter_map(|value| value.to_str().ok()).collect())
+    }
 }
 
 pub type HttpError = Box<dyn std::error::Error + Send + Sync + 'static>;
@@ -236,6 +246,8 @@
 
 #[cfg(test)]
 mod tests {
+    use http::HeaderValue;
+
     use super::*;
 
     #[test]
@@ -250,6 +262,32 @@
         )
     }
 
+    #[test]
+    fn http_headers_get_all() {
+        let mut carrier = http::HeaderMap::new();
+        carrier.append("headerName", HeaderValue::from_static("value"));
+        carrier.append("headerName", HeaderValue::from_static("value2"));
+        carrier.append("headerName", HeaderValue::from_static("value3"));
+
+        assert_eq!(
+            HeaderExtractor(&carrier).get_all("HEADERNAME"),
+            Some(vec!["value", "value2", "value3"]),
+            "all values from a key extraction"
+        )
+    }
+
+    #[test]
+    fn http_headers_get_all_missing_key() {
+        let mut carrier = http::HeaderMap::new();
+        carrier.append("headerName", HeaderValue::from_static("value"));
+
+        assert_eq!(
+            HeaderExtractor(&carrier).get_all("not_existing"),
+            None,
+            "all values from a missing key extraction"
+        )
+    }
+
     #[test]
     fn http_headers_keys() {
         let mut carrier = http::HeaderMap::new();

opentelemetry/src/propagation/mod.rs

@@ -40,6 +40,11 @@
 
     /// Collect all the keys from the underlying data.
     fn keys(&self) -> Vec<&str>;
+
+    /// Get all values from a key from the underlying data.
+    fn get_all(&self, key: &str) -> Option<Vec<&str>> {
+        self.get(key).map(|value| vec![value])
+    }
 }
 
 impl<S: std::hash::BuildHasher> Injector for HashMap<String, String, S> {
@@ -77,6 +82,30 @@
         );
     }
 
+    #[test]
+    fn hash_map_get_all() {
+        let mut carrier = HashMap::new();
+        carrier.set("headerName", "value".to_string());
+
+        assert_eq!(
+            Extractor::get_all(&carrier, "HEADERNAME"),
+            Some(vec!["value"]),
+            "case insensitive get_all extraction"
+        );
+    }
+
+    #[test]
+    fn hash_map_get_all_missing_key() {
+        let mut carrier = HashMap::new();
+        carrier.set("headerName", "value".to_string());
+
+        assert_eq!(
+            Extractor::get_all(&carrier, "missing_key"),
+            None,
+            "case insensitive get_all extraction"
+        );
+    }
+
     #[test]
     fn hash_map_keys() {
         let mut carrier = HashMap::new();