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/MappingLookup.java
* the provided mappings and set of mappers.
     * This is a commodity method to be used in tests, or whenever no mappings are defined for an index.
     * When creating a MappingLookup through this method, its exposed functionalities are limited as it does not
     * hold a valid {@link DocumentParser}, {@link IndexSettings} or {@link IndexAnalyzers}.
     *
     * @param mapping the mapping
     * @param mappers the field mappers
     * @param objectMappers the object mappers
     * @param aliasMappers the field alias mappers
     * @return the newly created lookup instance
     */
    public static MappingLookup fromMappers(
        Mapping mapping,
        Collection<FieldMapper> mappers,
        Collection<ObjectMapper> objectMappers,
        Collection<FieldAliasMapper> aliasMappers
    ) {
        return new MappingLookup(mapping, mappers, objectMappers, aliasMappers);
    }
    private MappingLookup(
        Mapping mapping,
        Collection<FieldMapper> mappers,
        Collection<ObjectMapper> objectMappers,
        Collection<FieldAliasMapper> aliasMappers
    ) {
        this.mapping = mapping;
        Map<String, Mapper> fieldMappers = new HashMap<>();
        Map<String, ObjectMapper> objects = new HashMap<>();
        List<NestedObjectMapper> nestedMappers = new ArrayList<>();
        for (ObjectMapper mapper : objectMappers) {