Alpine Add Package from Edge repository
You could specify the exact repo to apk, using the --repository parameter.
In your case:
apk add librdkafka --repository=http://dl-cdn.alpinelinux.org/alpine/edge/community
You can also add the repo:
echo "@testing http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories
and reference it with apk via @testing. Example:
apk add package-name@testing