[Stack Monitoring] add missing mb field

[estermv]

Summary

Fixes #110719

The bug was introduced in #96205 for 7.14, causing the ES query to be incorrect as we were missing the field in one of the aggregations:

Reviewer notes

• I'm not sure if this is the correct field from where we have to get the data. I used the same field as defined here because it has the same field.
• The field added is used here.

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: e898ef0
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Metrics [docs]

✅ unchanged

History

• 💔 Build #153514 failed 3384f34

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@elasticmachine merge upstream

[matschaffer]

Those +1,178,984 −240,770 diffs. They'll get you every time :)

Let me know if there are any testing steps you'd recommend, otherwise LGTM.