[8.19](backport #47299) [beatreceiver] only add default processors if running under agent

[mergify]

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)_
```<hr>This is an automatic backport of pull request #47299 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 78f115e has failed:

On branch mergify/bp/8.19/pr-47299
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 78f115e4f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   libbeat/beat/info.go
	modified:   libbeat/publisher/processing/default.go
	modified:   x-pack/filebeat/fbreceiver/factory.go
	modified:   x-pack/filebeat/fbreceiver/receiver_test.go
	modified:   x-pack/filebeat/tests/integration/otel_test.go
	modified:   x-pack/libbeat/cmd/instance/beat.go
	modified:   x-pack/libbeat/cmd/instance/beat_test.go
	modified:   x-pack/libbeat/outputs/otelconsumer/otelconsumer.go
	modified:   x-pack/libbeat/outputs/otelconsumer/otelconsumer_test.go
	modified:   x-pack/metricbeat/mbreceiver/factory.go
	modified:   x-pack/metricbeat/mbreceiver/receiver_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/metricbeat/tests/integration/otel_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @VihasMakwana? 🙏