from docs: https://docs.influxdata.com/telegraf/v1/install/

add telegraf sources:

curl --silent --location -O \\nhttps://repos.influxdata.com/influxdata-archive.key 
    && echo "943666881a1b8d9b849b74caebf02d3465d6beb716510d86a39f6c8e8dac7515  influxdata-archive.key"
    | sha256sum -c - && cat influxdata-archive.key
    | gpg --dearmor
    | sudo tee /etc/apt/trusted.gpg.d/influxdata-archive.gpg > /dev/null
    && echo 'deb [signed-by=/etc/apt/trusted.gpg.d/influxdata-archive.gpg] https://repos.influxdata.com/debian stable main'
    | sudo tee /etc/apt/sources.list.d/influxdata.list

install telegraf

sudo apt update && sudo apt install telegraf