[beatreceiver] only add default processors if running under agent

[VihasMakwana]

Proposed commit message

We're injecting default processors while running in standalone beatreceivers. This is not the case in standalone beats.
We should inject only if we're running under agent (Fixed via #46539).

Benchmarks shows improvement and are close to normal mode. There is still some difference and I'll work on that as a follow-up.

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.

How to test this PR locally

You can test the build from this PR with benchbuilder and compare the results without the changes of this PR.

Related issues

• Relates https://github.com/elastic/ingest-dev/issues/5823

Results

• Benchmark for standalone filebeat - normal mode.

### Benchmark summary table
| Environment ID | Race Id | Suite Name | Benchmark Name | Benchmark Type | Version | Duration (sec) | EPS | Store Bytes | Bytes/Event | Error |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| xyz | 8dd8bf0e-ef1b-4767-8c29-ad986afea6dd | logs-datastream | filestream-json-1024b-balanced | filebeat | `9.3.0` | `1m1.196885s` | `3268.000000` | `56872890` | `284.3644` |  |
| xyz | 43af615c-4b18-49b9-8ca2-4925c44644a3 | logs-datastream | filestream-json-1024b-throughput | filebeat | `9.3.0` | `45.795869s` | `4367.000000` | `56104542` | `280.5227` |  |
| xyz | d76fedb6-5352-4d62-969e-a7bc1aa14d3e | logs-datastream | filestream-json-1024b-scale | filebeat | `9.3.0` | `1m5.14271s` | `3070.000000` | `56834671` | `284.1734` |  |
| xyz | ae0a33c9-5032-47e3-a1e2-ddf0a8f3696d | logs-datastream | filestream-json-1024b-latency | filebeat | `9.3.0` | `19m12.550807s` | `174.000000` | `58669647` | `293.3482` |  |

_[Open in Kibana](https://ingest-metricstore.kb.us-central1.gcp.cloud.es.io:9243/app/dashboards#/view/ce699545-7f3f-42ce-b0f9-16c51e8b01ef?_g=%28time%3A%28from%3A%272025-10-25T14%3A27%3A07.052Z%27%2Cto%3A%272025-10-25T14%3A50%3A30.908Z%27%29%29&_a=%28filters%3A%21%28%28%27%24state%27%3A%28store%3AappState%29%2Cmeta%3A%28alias%3A%21n%2Cdisabled%3A%21f%2Cfield%3Aenvironment_id%2Cindex%3A%27e80ebe49-6c56-452d-a932-2e9026f9ff65%27%2Ckey%3Aenvironment_id%2Cnegate%3A%21f%2Cparams%3A%28query%3A%27xyz%27%29%2Ctype%3Aphrase%29%2Cquery%3A%28match_phrase%3A%28environment_id%3A%27xyz%27%29%29%29%29%2Cquery%3A%28language%3Akuery%2Cquery%3A%27%27%29%29)_

• Benchmark for filebeat otel mode (before my changes)

### Benchmark summary table
| Environment ID | Race Id | Suite Name | Benchmark Name | Benchmark Type | Version | Duration (sec) | EPS | Store Bytes | Bytes/Event | Error |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| xyz | 755ead19-ab37-4de9-a54c-9395ec17c43a | logs-datastream | filestream-json-1024b-balanced | filebeat | `9.3.0` | `1m56.138989s` | `1722.000000` | `105533417` | `527.6671` |  |
| xyz | 7a7e1caf-7a92-4396-b0c8-6050d6d40ca0 | logs-datastream | filestream-json-1024b-throughput | filebeat | `9.3.0` | `37.276624s` | `5365.000000` | `107132772` | `535.6639` |  |
| xyz | 467e93ba-5d28-4c1c-99f8-d7e83b84f100 | logs-datastream | filestream-json-1024b-scale | filebeat | `9.3.0` | `1m57.625477s` | `1700.000000` | `104303606` | `521.5180` |  |
| xyz | 16d50a55-4e64-4a30-b84c-64561fa7fb1e | logs-datastream | filestream-json-1024b-latency | filebeat | `9.3.0` | `20m8.961231s` | `165.000000` | `108167271` | `540.8364` |  |

_[Open in Kibana](https://ingest-metricstore.kb.us-central1.gcp.cloud.es.io:9243/app/dashboards#/view/ce699545-7f3f-42ce-b0f9-16c51e8b01ef?_g=%28time%3A%28from%3A%272025-10-25T20%3A12%3A12.283Z%27%2Cto%3A%272025-10-25T20%3A38%3A14.524Z%27%29%29&_a=%28filters%3A%21%28%28%27%24state%27%3A%28store%3AappState%29%2Cmeta%3A%28alias%3A%21n%2Cdisabled%3A%21f%2Cfield%3Aenvironment_id%2Cindex%3A%27e80ebe49-6c56-452d-a932-2e9026f9ff65%27%2Ckey%3Aenvironment_id%2Cnegate%3A%21f%2Cparams%3A%28query%3A%27xyz%27%29%2Ctype%3Aphrase%29%2Cquery%3A%28match_phrase%3A%28environment_id%3A%27xyz%27%29%29%29%29%2Cquery%3A%28language%3Akuery%2Cquery%3A%27%27%29%29)_

• Benchmark for filebeat otel mode (after my changes)

### Benchmark summary table
| Environment ID | Race Id | Suite Name | Benchmark Name | Benchmark Type | Version | Duration (sec) | EPS | Store Bytes | Bytes/Event | Error |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| xyz | 4eefb649-3ae4-498a-b46d-ad6b77f411b0 | logs-datastream | filestream-json-1024b-balanced | filebeat | `9.3.0` | `1m12.23267s` | `2769.000000` | `66011174` | `330.0559` |  |
| xyz | bfd54408-1075-405b-83f5-d20f06181a0e | logs-datastream | filestream-json-1024b-throughput | filebeat | `9.3.0` | `21.711077s` | `9212.000000` | `68151536` | `340.7577` |  |
| xyz | d9bab3f9-0628-480a-aee2-8e6107219e45 | logs-datastream | filestream-json-1024b-scale | filebeat | `9.3.0` | `1m12.787666s` | `2748.000000` | `65854534` | `329.2727` |  |
| xyz | 97cf4796-c3c6-400b-a5f5-e34d4a1f639e | logs-datastream | filestream-json-1024b-latency | filebeat | `9.3.0` | `19m17.321353s` | `173.000000` | `66767287` | `333.8364` |  |

_[Open in Kibana](https://ingest-metricstore.kb.us-central1.gcp.cloud.es.io:9243/app/dashboards#/view/ce699545-7f3f-42ce-b0f9-16c51e8b01ef?_g=%28time%3A%28from%3A%272025-10-25T19%3A46%3A19.037Z%27%2Cto%3A%272025-10-25T20%3A09%3A43.914Z%27%29%29&_a=%28filters%3A%21%28%28%27%24state%27%3A%28store%3AappState%29%2Cmeta%3A%28alias%3A%21n%2Cdisabled%3A%21f%2Cfield%3Aenvironment_id%2Cindex%3A%27e80ebe49-6c56-452d-a932-2e9026f9ff65%27%2Ckey%3Aenvironment_id%2Cnegate%3A%21f%2Cparams%3A%28query%3A%27xyz%27%29%2Ctype%3Aphrase%29%2Cquery%3A%28match_phrase%3A%28environment_id%3A%27xyz%27%29%29%29%29%2Cquery%3A%28language%3Akuery%2Cquery%3A%27%27%29%29)_

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @VihasMakwana? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[khushijain21]

This is going to break a lot of tests as we expect docuemnts indexed by standalone beats and beatreceivers to be same. And this PR changes the default processors to be available under agent only.

Also for managemt.Enabled() to return true, the beats process send "management.enabled=true" flag. But beatreceivers do not set this arg and hence will always result in false.

Have you tested this change with elastic-agent otel mode?

[VihasMakwana]

This is going to break a lot of tests as we expect docuemnts indexed by standalone beats and beatreceivers to be same. And this PR changes the default processors to be available under agent only.

Also for managemt.Enabled() to return true, the beats process send "management.enabled=true" flag. But beatreceivers do not set this arg and hence will always result in false.

For otel, we specify "management.otel.enabled=true" and that turns managemt.Enabled() flag. I have updated the tests. PTAL.

[elasticmachine]

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

[cmacknz]

Before merging this, open a PR in https://github.com/elastic/elastic-agent with the beats dependency updated to this branch and make sure there are no test failures introduced there.