docs: public static StubTransformer mappedCredentialsStubTransformer(
              final Map<String, CallCredentials> credentialsByName,
              @Nullable final CallCredentials fallback) {
          requireNonNull(credentialsByName, "credentials");
          return (name, stub) -> {
              final CallCredentials credentials = credentialsByName.getOrDefault(name, fallback);
              if (credentials == null) {
                  return stub;
              } else {
                  return stub.withCallCredentials(credentials);
              }
          };
      }