Current versions of Jira/Confluence
Based on changed APIs by Atlassian, external gadgets are no longer supported. Consequently, the approach described below no longer works, and traffic-light fields' content will not be rendered properly.
Instead, you can use my app "Integration of Jira fields into Confluence" (https://marketplace.atlassian.com/apps/1221429): since 2021, this app is archived on the Atlassian Marketplace and no longer available for installing due to economic reasons.
Nevertheless, you can install this app manually: download the app from my server at https://polscheit.de/plugins/jira/jiraFieldsInConfluence/jiraFieldsInConfluence-1.0.0_free_unsupported.jar and
install this JAR file via the button "upload" on your Confluence page "manage apps" as a system admin. This version is free of charge and unsupported at your own risk!
Please read the documentation and how to use at JFIC.
Besides rendering Jira fields including Traffic-Lights, you have also the advantage of specifying a Time Machine to use the field values as of <date/time>:
that's really helpful in case of e.g. reporting to always display the same historic value for a given timestamp.
...
For more details, please have a look on the app's documentation: JFIC
Older versions of Jira/Confluence
You can easily integrate your Jira issues into Confluence, but using the standard approach does not render your custom fields and just display their internal data as shown in the first section of the sample screenshot below.
...