How do I include prometheus-emitter in my helm chart?

A community member was trying to include prometheus-emitter in their helm chart, but, when deployed, received this error:

Exception in thread "main" Extension [/opt/druid/extensions/prometheus-emitter] specified in "druid.extensions.loadList" didn't exist!?

Community extensions need to be installed separately. In this particular example, the installation would look like:

cd /opt/druid
java \
  -cp "lib/*" \"extensions" \
  -Ddruid.extensions.hadoopDependenciesDir="hadoop-dependencies" \
  org.apache.druid.cli.Main tools pull-deps \
  --no-default-hadoop \
  -c "com.example:prometheus-emitter:0.xx.0"

Here’s some context.