small consistency fixes

Signed-off-by: eternal-flame-AD <yume@yumechi.jp>

src/lib.rs

@@ -822,7 +822,7 @@ pub struct App<C: UpstreamClient, S: Sandboxing> {
 pub fn register_cancel_handler() {
     #[cfg(target_family = "unix")]
     unsafe {
-        unsafe fn cancel_handler(_: libc::c_int) {
+        unsafe extern "C" fn cancel_handler(_: libc::c_int) {
             log::error!("Received cancel signal, stopping thread");
             libc::pthread_exit(&sandbox::EXIT_TIMEOUT as *const _ as *mut _);
         }

src/main.rs

@@ -241,6 +241,7 @@ fn main() {
                 return;
             }
 
+            #[cfg(feature = "metrics")]
             {
                 let reg = prometheus::default_registry();