Telemetry: send event report times as ISO strings

[angelapwen]

I found in the debug logs of my test run of the new event_report fields that the request was malformed due to the started_at and completed_at timestamps being sent as strings rather than ISO strings. There was no impact to any users as the feature flag this change was behind was off for all actors.

This change simply updates the timestamps so that they are sent with the appropriate type.

This time I have validated with test runs (links internally only): run 1, run 2 with debug logging and confirmed that I did not get an API error.

I also ran the query @esbena ran on the data warehouse and confirmed that we get 2 event_reports with length 1.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.