Treat session ID as a string in system/users

[fearful-symmetry]

What does this PR do?

We should be treating the session ID we get from DBus as a string:

      struct {
         string "106"
         uint32 1000
         string "alexk"
         string ""
         object path "/org/freedesktop/login1/session/_3106"
      }

The value seems to be an int 95% of the time, which is why I probably missed this. I'm still not sure under what circumstances we see a character thrown into the Session ID, but it does happen.

Why is it important?

If the Session ID isn't an int, we'll get an error.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Pull down and build
• On a linux/systemd box, run this metricset and insure that the system.users.id value is a string.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[kaiyan-sheng]

This change looks good. Missing changelog?

[elasticmachine]

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Build stats

• Build Cause: [Pull request #22359 updated]

• Start Time: 2020-11-02T21:39:28.910+0000

• Duration: 80 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2247
Skipped	508
Total	2755

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Test stats 🧪

Test	Results
Failed	0
Passed	2247
Skipped	508
Total	2755