[call_creds_util] Add debug checks for metadata accesses (#37822)

Fix https://github.com/grpc/grpc/issues/37701

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Closes #37822

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37822 from yijiem:debug-check-metadata 67df0da89c
PiperOrigin-RevId: 680734161

src/core/lib/security/credentials/call_creds_util.cc

@@ -18,6 +18,7 @@
 
 #include <string.h>
 
+#include "absl/log/check.h"
 #include "absl/log/log.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
@@ -42,6 +43,7 @@ struct ServiceUrlAndMethod {
 ServiceUrlAndMethod MakeServiceUrlAndMethod(
     const ClientMetadataHandle& initial_metadata,
     const grpc_call_credentials::GetRequestMetadataArgs* args) {
+  DCHECK(initial_metadata->get_pointer(HttpPathMetadata()) != nullptr);
   auto service =
       initial_metadata->get_pointer(HttpPathMetadata())->as_string_view();
   auto last_slash = service.find_last_of('/');
@@ -56,6 +58,7 @@ ServiceUrlAndMethod MakeServiceUrlAndMethod(
     method_name = service.substr(last_slash + 1);
     service = service.substr(0, last_slash);
   }
+  DCHECK(initial_metadata->get_pointer(HttpAuthorityMetadata()) != nullptr);
   auto host_and_port =
       initial_metadata->get_pointer(HttpAuthorityMetadata())->as_string_view();
   absl::string_view url_scheme = args->security_connector->url_scheme();