PMM-14215: Fix OS metrics for PMM dump

[idoqo]

https://jira.percona.com/browse/PMM-PMM-14215

• Links to relevant pull requests

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/3403/

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4001-d6aa8fe
Client docker: perconalab/pmm-client-fb:PR-4001-d6aa8fe
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4001-d6aa8fe
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4001-d6aa8fe.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4001-d6aa8fe&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4001-d6aa8fe.tar.gz

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/3419/

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4001-ae58ac7
Client docker: perconalab/pmm-client-fb:PR-4001-ae58ac7
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4001-ae58ac7
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4001-ae58ac7.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4001-ae58ac7&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4001-ae58ac7.tar.gz

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/3473/