add benchmark input to filebeat spec file

[leehinman]

What does this PR do?

Adds the filebeat benchmark input (added for 8.14) to the filebeat spec file

Why is it important?

Without this integrations can not be written that use the benchmark input.

Checklist

• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Write an integration that uses the benchmark input

elastic/integrations#8733

Related issues

• Relates filebeat benchmark input and discard output beats#37437

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[mergify]

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

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

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

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[cmacknz]

You want this to go in https://github.com/elastic/beats/blob/main/x-pack/agentbeat/agentbeat.spec.yml

I started the work to remove these to avoid this confusion but it isn't done yet.

[leehinman]

Closing in favor of elastic/beats#39789