Solr - Add extra fields to the Sitecore_analytics_index to store custom xdb data

Solr.CustomFieldsAnalyticsFields.xml

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <configuration>
        <indexes>
          <index id="sitecore_analytics_index">
            <configuration>
              <fieldMap>
                <fieldNames>
                  <field fieldName="contact.SampleOrder.Skus" returnType="stringCollection" />
                  <field fieldName="contact.SampleOrder.FavouriteType" returnType="string" />
                </fieldNames>
              </fieldMap>
            </configuration>
          </index>
        </indexes>
      </configuration>
    </contentSearch>
  </sitecore>
</configuration>