Also decode for headers passed via the API

opentelemetry-otlp/src/exporter/http/mod.rs

@@ -120,7 +120,7 @@ impl HttpExporterBuilder {
     pub fn with_headers(mut self, headers: HashMap<String, String>) -> Self {
         // headers will be wrapped, so we must do some logic to unwrap first.
         let mut inst_headers = self.http_config.headers.unwrap_or_default();
-        inst_headers.extend(headers);
+        inst_headers.extend(headers.map(|(k, v)| (k, urlencoding::decode(v).unwrap_or_default())));
         self.http_config.headers = Some(inst_headers);
         self
     }