Fix modelindexer benchmarks

[axw]

Fix the modelindexer benchmarks, which were failing due to the mock Elasticsearch server not returning the appropriate X-Elastic-Product header.

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[ghost]

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Build stats

• Start Time: 2022-03-15T02:14:29.246+0000

• Duration: 48 min 42 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4382
Skipped	14
Total	4396

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