[Tracing] Instrument code to emit Task Execution events

[ViniciustCosta]

Description

This PR instruments the code to emit TaskExecutionEvents (defined at #4857) during utasks execution (events for trusted tasks will be added in a next PR).

The implementation leverages the _MetricRecorder from utasks module, since it already wraps the execution of all task stages while also storing important metadata from the task. This enables us sending events with context for each stage, even if unhandled exceptions happen during runtime.

Events are emitted in the following cases:

• After Preprocess executed:
  • If an exception occurs: Event: {TaskStatus=Exception, Outcome=Unhandled exception}
  • If uworker input is NOT returned: Event: {TaskStatus=Exception, Outcome=No preprocess return}
  • Else preprocess is successful: Event: {TaskStatus=Started, Outcome=None}
• At the start of Postprocess:
  • States that main executed: Event: {TaskStatus=Postprocess Started, Outcome=<uworker_output.error_type>}
• After Postprocess executed:
  • If an exception occurs: Event: {TaskStatus=Exception, Outcome=Unhandled exception}
  • Else postprocess finishes successfully: Event: {TaskStatus=Postprocess Completed, Outcome=<uworker_output.error_type>}

Notice that a known limitation is that we do not send events from utask main, since we currently don't have a workaround to access datastore from untrusted workers.

b/394059581

Tests

Unit tests were added to verify all the instrumented codepaths and check the correctness of the events being emitted.

Runtime tests were done by deploying to dev environment.

• Task execution events created:

• No new error groups related to the events: Error groups

[vitorguidi]

Is this being used?

[ViniciustCosta]

Sorry, I merged this before seeing this comment. It is not being used, I'll remove in a further PR