cleanup(otel): silence clang-tidy in C++20 mode (#13010)

As I write more tests and examples in C++20 code in headers gets
examined by `clang-tidy` in C++20 mode. In this case, `clang-tidy` would
like us to use C++20 ranges to perform a reverse iteration.

Author: coryan

google/cloud/internal/opentelemetry_context.h

@@ -109,6 +109,7 @@ class ScopedOTelContext {
 
   ~ScopedOTelContext() {
     if (noop_) return;
+    // NOLINTNEXTLINE(modernize-loop-convert)
     for (auto it = contexts_.rbegin(); it != contexts_.rend(); ++it) {
       DetachOTelContext(*it);
     }