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 the collection of fields that have been ignored so far.
     */
    public final Collection<String> getIgnoredFields() {
        return Collections.unmodifiableCollection(ignoredFields);
    }
    /**
     * Add the given {@code field} to the _field_names field
     *
     * Use this if an exists query run against the field cannot use docvalues
     * or norms.
     */
    public final void addToFieldNames(String field) {
        FieldNamesFieldMapper fieldNamesFieldMapper = (FieldNamesFieldMapper) getMetadataMapper(FieldNamesFieldMapper.NAME);
        if (fieldNamesFieldMapper != null) {
            fieldNamesFieldMapper.addFieldNames(this, field);
        }
    }
    public final Field version() {
        return this.version;
    }
    public final void version(Field version) {
        this.version = version;
    }
    public final String id() {
        if (id == null) {
            assert false : "id field mapper has not set the id";
            throw new IllegalStateException("id field mapper has not set the id");
        }