[8.x](backport #41939) Add unique per beat monitoring namespace

[mergify]

Proposed commit message

Add a unique per beat monitoring namespace. If this is nil then the current default namespace is used. This is necessary so a Beat receiver can be instantiated multiple times without having panics due to registering the same variable twice.

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

None, behavior is unchanged for normal beats usage.

Author's Checklist

• [ ]

How to test this PR locally

Unit tests

Run the unit test suites to make sure no behavior changes for normal beats usage

By hand

1. Build elastic-agent using this PR to satisfy the beat dependency
2. Run `elastic-agent otel -c otel.yml with the following config

receivers:
  filebeatreceiver/1:
    filebeat:
      inputs:
        - type: benchmark
          enabled: true
          message: "test"
          eps: 1
    output:
      otelconsumer:
    logging:
      level: debug
      selectors:
        - '*'
    processors:
      - add_fields:
          target: project
          fields:
            name: filebeatreceiver1
  filebeatreceiver/2:
    filebeat:
      inputs:
        - type: benchmark
          enabled: true
          message: "test"
          eps: 1
    output:
      otelconsumer:
    logging:
      level: debug
      selectors:
        - '*'
    processors:
      - add_fields:
          target: project
          fields:
            name: filebeatreceiver2
  metricbeatreceiver/1:
    metricbeat:
      modules:
        - module: system
          metricsets:
            - cpu
          count: 1
          enabled: true
          period: 1s
    output:
      otelconsumer:
    logging:
      level: debug
      selectors:
        - '*'
    processors:
      - add_fields:
          target: project
          fields:
            name: metricbeatreceiver1
  metricbeatreceiver/2:
    metricbeat:
      modules:
        - module: system
          metricsets:
            - cpu
          count: 1
          enabled: true
          period: 1s
    output:
      otelconsumer:
    logging:
      level: debug
      selectors:
        - '*'
    processors:
      - add_fields:
          target: project
          fields:
            name: metricbeatreceiver2
exporters:
  file/1:
    path: /path/to/log1.txt
  file/2:
    path: /path/to/log2.txt

service:
  pipelines:
    logs/1:
      receivers:
        - filebeatreceiver/1
        - metricbeatreceiver/1
      exporters:
        - file/1
    logs/2:
      receivers:
        - filebeatreceiver/2
        - metricbeatreceiver/2
      exporters:
        - file/2

All 4 receivers should be running, with events going to separate log files.

Related issues

• Closes RegisterHostname causes panic if 2 filebeat receivers are configured #41743

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #41939 done by [Mergify](https://mergify.com).

[botelastic]

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