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
protected abstract void addDoc(LuceneDocument doc);
    /**
     * Find a dynamic mapping template for the given field and its matching type
     *
     * @param fieldName the name of the field
     * @param matchType the expecting matchType of the field
     * @return the matching template; otherwise returns null
     * @throws MapperParsingException if the given field has a dynamic template name specified, but no template matches that name.
     */
    public final DynamicTemplate findDynamicTemplate(String fieldName, DynamicTemplate.XContentFieldType matchType) {
        final String pathAsString = path().pathAsText(fieldName);
        final String matchTemplateName = sourceToParse().dynamicTemplates().get(pathAsString);
        for (DynamicTemplate template : root().dynamicTemplates()) {
            if (template.match(matchTemplateName, pathAsString, fieldName, matchType)) {
                return template;
            }
        }
        if (matchTemplateName != null) {
            throw new MapperParsingException(
                "Can't find dynamic template for dynamic template name [" + matchTemplateName + "] of field [" + pathAsString + "]"
            );
        }
        return null;
    }
    
    
    private static class CopyToParser extends FilterXContentParserWrapper {
        enum State {
            FIELD,
            VALUE