fix: harden typed metrics review findings

crates/crank-core/tests/correlation.rs

@@ -55,6 +55,8 @@ fn traceparent_parser_is_strict_and_never_accepts_zero_ids() {
         "0af7651916cd43dd8448eb211c80319c"
     );
     assert_eq!(context.traceparent(), VALID_TRACEPARENT);
+    assert!(context.is_sampled());
+    assert!(!TraceContext::generate().is_sampled());
 
     for invalid in [
         "00-00000000000000000000000000000000-b7ad6b7169203331-01",