You can configure a system to make use of this repository, thus: 
$ sudo tee /etc/apt/sources.list.d/salsa-kenhys-rrdtool-9136907.sources >/dev/null <<EOF
Types: deb
URIs: https://salsa.debian.org/kenhys/rrdtool/-/jobs/9136907/artifacts/raw/aptly
Suites: unstable
Components: main
Signed-By:
 -----BEGIN PGP PUBLIC KEY BLOCK-----
 .
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 XP0Uqm5glzQ5Xu02t4x0C02AjbcOx45UUdtAnuz8OXgRdUEBP9afQgpjrX5eZLnC
 UrccqmNUn23/z3uOYAYvcRWTpJTtqzrsVwwlABEBAAG0KlNhbHNhIFBpcGVsaW5l
 IDxzYWxzYS1waXBlbGluZUBkZWJpYW4ub3JnPokBTwQTAQoAORYhBNI6IrQG+ksN
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 =4Mgh
 -----END PGP PUBLIC KEY BLOCK-----
EOF

Alternatively, using the old one-line format, thus: 
$ wget -O - https://salsa.debian.org/kenhys/rrdtool/-/jobs/9136907/artifacts/raw/aptly/public-key.asc | sudo tee /etc/apt/keyrings/salsa-kenhys-rrdtool-9136907.asc >/dev/null
$ sudo tee /etc/apt/sources.list.d/salsa-kenhys-rrdtool-9136907.list >/dev/null <<EOF
deb [signed-by=/etc/apt/keyrings/salsa-kenhys-rrdtool-9136907.asc] https://salsa.debian.org/kenhys/rrdtool/-/jobs/9136907/artifacts/raw/aptly unstable main
# deb-src [signed-by=/etc/apt/keyrings/salsa-kenhys-rrdtool-9136907.asc] https://salsa.debian.org/kenhys/rrdtool/-/jobs/9136907/artifacts/raw/aptly unstable main
EOF

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