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
* Returns an iterable over all the registered field mappers (including alias mappers)
     */
    public Iterable<Mapper> fieldMappers() {
        return fieldMappers.values();
    }
    /**
     * Gets the postings format for a particular field
     * @param field the field to retrieve a postings format for
     * @return the postings format for the field, or {@code null} if the default format should be used
     */
    public PostingsFormat getPostingsFormat(String field) {
        return completionFields.contains(field) ? CompletionFieldMapper.postingsFormat() : null;
    }
    void checkLimits(IndexSettings settings) {
        checkFieldLimit(settings.getMappingTotalFieldsLimit());
        checkObjectDepthLimit(settings.getMappingDepthLimit());
        checkFieldNameLengthLimit(settings.getMappingFieldNameLengthLimit());
        checkNestedLimit(settings.getMappingNestedFieldsLimit());
        checkDimensionFieldLimit(settings.getMappingDimensionFieldsLimit());
    }
    private void checkFieldLimit(long limit) {
        checkFieldLimit(limit, 0);
    }
    void checkFieldLimit(long limit, int additionalFieldsToAdd) {
        if (fieldMappers.size() + objectMappers.size() + additionalFieldsToAdd - mapping.getSortedMetadataMappers().length > limit) {
            throw new IllegalArgumentException(
                "Limit of total fields ["
                    + limit
                    + "] has been exceeded"
                    + (additionalFieldsToAdd > 0 ? " while adding new fields [" + additionalFieldsToAdd + "]" : "")