How to fix the following signatures couldnt be verified because the public key is not available with apt

I was trying to install grafana on ubuntu when I got faced with: "the following signatures couldn't be verified because the public key is not available" error as seen below:

$ sudo add-apt-repository "deb https://packages.grafana.com/oss/deb stable main"
Hit:1 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic InRelease
Get:2 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:3 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:4 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:5 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-backports/universe Sources [2068 B]
Get:6 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-backports/universe amd64 Packages [3492 B]
Get:7 https://packages.grafana.com/oss/deb stable InRelease [12.1 kB]
Err:7 https://packages.grafana.com/oss/deb stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C8C34C524098CB6
Reading package lists... Done

In order to continue, we need to import the trusted key:

$ apt-key adv --keyserver keyserver.ubuntu.com --recv-keys  8C8C34C524098CB6
Executing: /tmp/apt-key-gpghome.9xlwQh2M06/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 8C8C34C524098CB6
gpg: key 8C8C34C524098CB6: public key "Grafana <info@grafana.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1

Now that the key has been imported, we can update and continue:

$ apt update
Hit:1 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:3 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
Get:5 https://packages.grafana.com/oss/deb stable InRelease [12.1 kB]
Get:6 https://packages.grafana.com/oss/deb stable/main amd64 Packages [10.8 kB]
Fetched 22.9 kB in 1s (32.7 kB/s)
Reading package lists... Done

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