Java attacher support for macOS

[eyalkoren]

The Java attacher discovery relies on go-sysinfo, which requires cgo for macOS, which is not built into APM Server.
Optional alternatives:

• enhance go-sysinfo to use sysctl for listing processes, similar to https://github.com/mitchellh/go-ps
• implement discovery for darwin based on ps execution (we had this working already)
• repackage cgo

[marclop]

I've started looking into changing go-sysinfo to use a non cgo implementation to gather the required information for Darwin. It looks like the latest published tags for golang.org/x/sys expose a more complete API to perform sysctl and syscall without relying on cgo or syscall.syscall6 calls.

[marclop]

I'm going to move this back to the Ready column as I haven't yet started implementing the necessary changes in go-sysinfo.

[eyalkoren]

Once the discovery part is handled, we will still need to adjust the per-user tmp dir copy. In macOS, the Go utility to create temp dir (or find tmp dir root) would return a path to a directory which is specific to the current user (i.e. the APM Server user).
AFAIK, the functionality to run as a different user we already have in place is applicable to macOS, which means that we can run echo $TMPDIR as the target user and read the command output to reveal the user-specific temp folder.

[simitt]

Tracking the removal of the technical blockers in #9185

[simitt]

Reopening until #8718 (comment) is addressed.

[eyalkoren]

MacOS is mostly supported now without relying on cgo.
However, the process start time is not supported yet, so we'll need to find an alternative to identify a process instance, which currently relies on PID+start-time.