Implement OTLP export integ tests

[JayPolanco]

Description of the issue

Add integration tests for the OTLP export feature, which allows the CloudWatch Agent to export metrics directly to the CloudWatch monitoring endpoint via OTLP/HTTP with SigV4 authentication.

Description of changes

• Add three EC2 integration tests validating OTLP export for hostmetrics, statsd, and collectd receivers
• Add shared OTLP metrics query utility (otlpmetricsquery.go) that queries the CloudWatch Prometheus-compatible API with SigV4 signing
• Add shared validation helper (otlpvalidation/validate.go) for PromQL-based metric verification
• Add OTEL YAML agent configs for each test with otlphttp exporter, sigv4auth, resourcedetection, and batch processing
• Add test matrix generator entries for the new test suite
• Tests use native OTEL YAML configs (not CWA JSON), started via agent-ctl -a fetch-config -s -c file:/tmp/config.yaml

License

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms
of your choice.

Tests

All three tests pass in CI via the EC2LinuxOTLPExportTest workflow job (us-west-2, AL2, amd64). Manually validated on EC2 that metrics flow end-to-end: receiver → pipeline → otlphttp exporter → monitoring endpoint → PromQL query returns results, we validate all metrics were published successfully.

https://github.com/aws/amazon-cloudwatch-agent/actions/runs/24367971739