fix: use collect() instead of for loop

opentelemetry-otlp/tests/grpc_build.rs

@@ -44,13 +44,13 @@ fn build_content_map() -> HashMap<PathBuf, String> {
     let dict =
         std::fs::read_dir("src/proto/grpcio").expect("cannot open dict of generated grpc files");
 
-    for entry in dict.into_iter().flatten() {
+    let _ = dict.into_iter().flatten().map(|entry|{
         map.insert(
             entry.path(),
             std::fs::read_to_string(entry.path()).unwrap_or_else(|_| {
                 panic!("cannot read from file {}", entry.path().to_string_lossy())
             }),
         );
-    }
+    }).collect::<Vec<()>>();
     map
 }