public String functionName(String method) {
        final ThriftFunction function = metadata.function(method);
        requireNonNull(function, "metadata.function(method) returned null. metadata: " + metadata +
                                 ", method: " + method);
        return function.name();
    }
Returns the associated thrift method name in the Thrift IDL.
thrift/thrift0.13/src/main/java/com/linecorp/armeria/server/thrift/ThriftServiceEntry.java