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
*/
    public final RootObjectMapper.Builder updateRoot() {
        return mappingLookup.getMapping().getRoot().newBuilder(indexSettings.getIndexVersionCreated());
    }
    public boolean isWithinCopyTo() {
        return false;
    }
    /**
     * Return a new context that will be used within a nested document.
     */
    public final DocumentParserContext createNestedContext(String fullPath) {
        if (isWithinCopyTo()) {
            
            return this;
        }
        final LuceneDocument doc = new LuceneDocument(fullPath, doc());
        addDoc(doc);
        return switchDoc(doc);
    }
    /**
     * Return a new context that has the provided document as the current document.
     */
    public final DocumentParserContext switchDoc(final LuceneDocument document) {
        return new Wrapper(this) {
            @Override
            public LuceneDocument doc() {
                return document;
            }
        };
    }