fix(opentelemetry source): count individual items in OTLP batches for component_received_events_total metric

[thomasqueirozb]

Summary

Important context: #24316 (comment)

Fixes event counting in the OpenTelemetry source when use_otlp_decoding: true. The component_received_events_total metric now correctly counts individual log records, metrics, and spans within OTLP batches instead of counting the number of batch requests.

This ensures consistency with:

• Other Vector sources (Splunk HEC, Datadog, Kafka)
• The same OpenTelemetry source when use_otlp_decoding: false

Since the previous fix was never released this one is being marked as no-changelog

Vector configuration

sources:
  source0:
    type: opentelemetry
    grpc:
      address: 0.0.0.0:4317
    http:
      address: 0.0.0.0:4318
    use_otlp_decoding: true

How did you test this PR?

Added comprehensive e2e tests that verify the metric counts individual items:

• vector_component_received_events_total_counts_individual_log_records (logs)
• vector_component_received_events_total_counts_individual_metrics (metrics)

Ran both e2e test suites:

./scripts/run-integration-test.sh e2e opentelemetry-logs
./scripts/run-integration-test.sh e2e opentelemetry-metrics

Both test suites passed, confirming the fix correctly counts 200 log records and 400 metrics instead of batch requests.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: Missing events count when switching to "use_otlp_decoding: true" #24316
• Related: fix(opentelemetry source): emit component_received_event* metrics when use_otlp_decoding is enabled #24480

[thomasqueirozb]

Skipping the changelog as this is a fixup to the other one