echo "deb http://repo.logdna.com stable main" | sudo tee /etc/apt/sources.list.d/logdna.list
    sudo apt-get update
    sudo apt-get install -y --force-yes
    logdna-agent sudo logdna-agent -k 6a7b7c622290d1a49bbe5a94dc6828d

Haven't tried this yet but the install instructions don't include adding a GPG key. Instead it has the option "--force-yes" which I believe skips that check.

That would work for the initial install but wouldn't it complain down the road when you try to update the package?