[exporter/datadog] support setting the source field

[sudomateo]

Component(s)

exporter/datadog

Is your feature request related to a problem? Please describe.

I wrote up everything in DataDog/opentelemetry-mapping-go#355 since that's where the feature would need to be implemented but Datadog Support requested that I open an issue on this repository instead. I'm going out of office starting tomorrow so I wanted to get this in beforehand. I'll come back and transfer the content over nicely to this issue when I'm back.

Describe the solution you'd like

The following configuration should correctly set the source field within Datadog.

processors:
  transform:
    log_statements:
      - context: log
        statements:
          - set(attributes["ddtags"], "source:sudomateo")

Describe alternatives you've considered

N/A

Additional context

N/A

[github-actions]

Pinging code owners:

• exporter/datadog: @mx-psi @dineshg13 @liustanley @songy23 @mackjmr @ankitpatel96

See Adding Labels via Comments if you do not have permissions to add labels yourself.