[9.0] (backport #7754) Set fullnameOverride and validate Otel kube-stack deployment datastreams are being written (metrics, traces)

[mergify]

What does this PR do?

Adds tests to ensure the Helm chart is correctly deployed and data pushed to Elasticsearch:

• Validates metrics collected with the kubeletstats receiver are written to Elasticsearch.
• Deploys a Java application to validate auto-instrumentation works properly and the corresponding traces documents are available in Elasticsearch. As an alternative, I tried to install the OpenTelemetry demo Helm Chart and assert its traces, but it is too heavy (big images, e.g. flagd → 1.2GB) for the testing timeouts.

Why is it important?

Fixes #7381

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This is an automatic backport of pull request #7754 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 7e6ad5f

Failed CI Steps

• Packaging: Ubuntu arm64 FIPS

cc @rogercoll