Revert "let the system/process metricset monitor a single PID (#39620)"

[pchila]

Proposed commit message

This reverts commit 43d5dd4 introduced with PR #39620.

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• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @pchila? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

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[rdner]

The failure seems to be caused by updating github.com/coreos/go-systemd/v22

https://github.com/coreos/go-systemd/blob/7d375ecc2b092916968b5601f74cca28a8de45dd/internal/dlopen/dlopen_example.go#L54

according to our error message:

dlopen_example.cgo2.c:(.text+0x36): multiple definition of `my_strlen'; /tmp/go-link-1331529901/000031.o:dlopen_example.cgo2.c:(.text+0x36): first defined here

[pchila]

The failure seems to be caused by updating github.com/coreos/go-systemd/v22

https://github.com/coreos/go-systemd/blob/7d375ecc2b092916968b5601f74cca28a8de45dd/internal/dlopen/dlopen_example.go#L24

according to our error message:

dlopen_example.cgo2.c:(.text+0x36): multiple definition of `my_strlen'; /tmp/go-link-1331529901/000031.o:dlopen_example.cgo2.c:(.text+0x36): first defined here

Interesting... in go.mod and go.sum of PR #39620 we can see that go-systemd has been updated

And in go.sum we see 2 versions of go-systemd but to have a duplicate definition we need to be linking the same symbol twice so:

• either the module is compiled twice (2 diff versions) and linked twice
• some part of agent beat includes the same module multiple times from multiple beats

[andrewkroh]

coreos/go-systemd copied the code that includes this symbol from coreos/pkg. We use both. So now we have a conflict.

The symbol in question is purely test code so it should have been included in a file with a _test.go suffix.

Haven't had any coffee yet so double check my work 😄 .