[9.0](backport #45150) x-pack/filebeat/input/gcppubsub: fix integration tests

[mergify]

Proposed commit message

This PR addresses two issues in the GCP Pub/Sub input tests:

1. GCP Pub/Sub Emulator Container Missing:

Tests like TestTopicDoesNotExist were being skipped due to the missing PUBSUB_EMULATOR_HOST environment variable:

<testcase classname="github.com/elastic/beats/v7/x-pack/filebeat/input/gcppubsub" name="TestTopicDoesNotExist" time="0.000000">
<skipped message="=== RUN TestTopicDoesNotExist pubsub_test.go:230: PUBSUB_EMULATOR_HOST is not set in environment. You can start the emulator with "docker-compose up" from the _meta directory. The default address is PUBSUB_EMULATOR_HOST=localhost:8432 --- SKIP: TestTopicDoesNotExist (0.00s) "/>
</testcase>

This has been fixed by ensuring the Pub/Sub emulator Docker container is properly launched for the test environment.

2. Fix for Segfault in NewInput.func2:

A segmentation fault occurred when test code accessed in.metrics before it was initialized.

=== FAIL: x-pack/filebeat/input/gcppubsub TestSubscriptionExists (unknown)
    logger.go:146: 2025-06-26T17:56:37.058+0200	INFO	gcp.pubsub	logp/logger.go:169	Initialized GCP Pub/Sub input.	{"pubsub_project": "test-project-id", "pubsub_topic": "test-topic-foo", "pubsub_subscription": {"Name":"test-subscription-bar","NumGoroutines":1,"MaxOutstandingMessages":1600,"Create":true}}
    pubsub_test.go:142: Topic created: test-topic-foo
    pubsub_test.go:189: New subscription created: test-subscription-bar
    pubsub_test.go:166: Published 5 messages to topic test-topic-foo. ID range: [1, 5]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x8 pc=0x1056a2740]

Metrics are now explicitly initialized in the test setup to avoid nil pointer dereferencing.

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.

How to test this PR locally

Run the integration tests from x-pack/filebeat with:

# mage -v goIntegTest

---

This is an automatic backport of pull request #45150 done by [Mergify](https://mergify.com).

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[mergify]

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