fix(aggregate transform): set interval_ms on incremental counters

[thomasqueirozb]

Summary

Fixes #23762 where counter metrics from the aggregate transform were sent to Datadog as counts instead of rates. The aggregate transform now sets interval_ms on incremental counter metrics to match the configured flush interval.

Vector configuration

sources:
  demo:
    type: demo_logs
    format: apache_common
    interval: 1.0

transforms:
  metrics:
    type: log_to_metric
    inputs: [demo]
    metrics:
      - type: counter
        field: message
        name: requests

  aggregate:
    type: aggregate
    inputs: [metrics]
    interval_ms: 30000

sinks:
  datadog:
    type: datadog_metrics
    inputs: [aggregate]
    default_api_key: "${DATADOG_API_KEY}"

How did you test this PR?

• Added unit tests for setting interval_ms on counters
• Added unit tests for NOT setting interval_ms on gauges
• All 16 aggregate tests pass
• All 19 Datadog encoder tests pass

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

• Closes: Rate counters being sent as counters in Datadog sink #23762