We have a check that enforces the total number of fields needs to be below a certain (configurable) threshold in each index. 

Runtime fields currently do not contribute to the count. There have been misunderstandings around this, caused by runtime fields not having a corresponding lucene field. Object fields and alias fields don't have corresponding lucene fields either yet they count towards the total fields limit.

The original goal of the total fields limit is to prevent a mappings explosion caused by a bogus document being indexed, as it would cause fields being dynamically added to the index mappings, which hinders stability due to the cluster state size of mappings as well as memory footprint of mappings in data nodes.

Runtime fields contribute to cluster state size and memory footprint of mappings like any other field, and they can be created dynamically when [`dynamic` is set to `runtime`](https://www.elastic.co/guide/en/elasticsearch/reference/current/dynamic.html#dynamic-parameters). Currently a bogus document can cause a mappings explosion when `dynamic` is set to `runtime` which is a bug. For this reason runtime fields should be counted towards the total fields count.
server/src/main/java/org/elasticsearch/index/mapper/DocumentParserContext.java
/**
     * Return a context for copy_to directives
     * @param copyToField   the name of the field to copy to
     * @param doc           the document to target
     */
    public final DocumentParserContext createCopyToContext(String copyToField, LuceneDocument doc) throws IOException {
        ContentPath path = new ContentPath(0);
        XContentParser parser = DotExpandingXContentParser.expandDots(new CopyToParser(copyToField, parser()), path::isWithinLeafObject);
        return new Wrapper(this) {
            @Override
            public ContentPath path() {
                return path;
            }
            @Override
            public XContentParser parser() {
                return parser;
            }
            @Override
            public boolean isWithinCopyTo() {
                return true;
            }
            @Override
            public LuceneDocument doc() {
                return doc;
            }
        };
    }
    /**
     *  @deprecated we are actively deprecating and removing the ability to pass
     *              complex objects to multifields, so try and avoid using this method