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