You can configure a system to make use of this repository, thus: 
$ sudo tee /etc/apt/sources.list.d/salsa-rrdtool-team-rrdtool-8859021.sources >/dev/null <<EOF
Types: deb
URIs: https://salsa.debian.org/rrdtool-team/rrdtool/-/jobs/8859021/artifacts/raw/aptly
Suites: unstable
Components: main
Signed-By:
 -----BEGIN PGP PUBLIC KEY BLOCK-----
 .
 mQENBGlfgXsBCADdJWBmV+xfTR7cCbGtkjD9NQwp7G5szAC7HKAYkLRZjfUTnSQW
 wdBWmTiK5JeDDth5m15QfCHPXjm7ndTC60HlUVwgK19eckHF8lD9h0JipqPz4s6+
 8JMoRUlowoBiuShyKag/MKmJ2Z05UmJAowQmw9UFC42/VdkDcAlkkbCzJ6RXhaN5
 nQit9QfxdXKFoSG9nzv9vyr4RcGKD/4l3qaj/gJy4WzgGjRi2VJ23/hB1l8+li1M
 T1j2p2FKThb+mXs0BN7LN4CpEk/3F8PQMZiLAT/AN8FLDG8VNQtqi59lhqlmPjzU
 pmbvp4Q/tySIYyTQGDtGzW83Hdmqbgg3Q5dBABEBAAG0KlNhbHNhIFBpcGVsaW5l
 IDxzYWxzYS1waXBlbGluZUBkZWJpYW4ub3JnPokBTwQTAQoAORYhBFDepMx/KcEJ
 YJ5bvDt0NBQ3vo1pBQJpX4F7AxsvBAULCQgHAgYVCgkICwIEFgIDAQIeAQIXgAAK
 CRA7dDQUN76NaenfB/sEUu/5UYgbxpjdOj+sAMyZDh7QlYQP7kNZCHMMQM/x+7BQ
 08K4BG7WOZtU4XJdIUK4iI53qpqjFOyussLh2o1C071E5fGWeouemIF3XfThfda2
 kePHtCSawWhjLGH4TNk+wU3ek1LyKgYr1U5Weky5rZzMJVBAVWtiZmspPXbVFKXJ
 ozN38/zoOKXoJqIW/ajlhgwqshV+gUqkVsHnt+sNKMUiEWUQLfGbG12K0xPDfEqI
 dvSDW7fpkT8wSELthtuwLC5+KeRJdzo+N2Z/7uI2jLHCPMC3gQUWEdRlTZExqtO9
 TwheLEvF/KKGaEwj5zAdxNAbL8fgjlHz4YMEroKe
 =yssN
 -----END PGP PUBLIC KEY BLOCK-----
EOF

Alternatively, using the old one-line format, thus: 
$ wget -O - https://salsa.debian.org/rrdtool-team/rrdtool/-/jobs/8859021/artifacts/raw/aptly/public-key.asc | sudo tee /etc/apt/keyrings/salsa-rrdtool-team-rrdtool-8859021.asc >/dev/null
$ sudo tee /etc/apt/sources.list.d/salsa-rrdtool-team-rrdtool-8859021.list >/dev/null <<EOF
deb [signed-by=/etc/apt/keyrings/salsa-rrdtool-team-rrdtool-8859021.asc] https://salsa.debian.org/rrdtool-team/rrdtool/-/jobs/8859021/artifacts/raw/aptly unstable main
# deb-src [signed-by=/etc/apt/keyrings/salsa-rrdtool-team-rrdtool-8859021.asc] https://salsa.debian.org/rrdtool-team/rrdtool/-/jobs/8859021/artifacts/raw/aptly unstable main
EOF

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