Verify datadog_events sink

[neuronull]

• Verify internal event processing metrics are tested via assert_X_compliance test helpers
• Update component_discarded_events_total to match the new instrumentation spec by adding reason to the emitted logs, and intentional to emitted logs and metrics
• Verify that the component correctly emits the correct error events per the component spec

[neuronull]

Notes:

• some changes to unit tests, apply happy path assert helpers to each happy path test case
• blocked by enhancement(datadog_logs sink): Adhere to instrumentation spec #14225 , as content from that commit will be applicable here. Have made TODOs on branch.