profiler: record timestamp at the start execution tracing

[nsrip-dd]

What does this PR do?

Sets the timestamp for the last execution trace recording at the beginning,
rather than the end, of recording.

Motivation

Setting a tracing period matching the profile period will do the expected thing
of sending a trace with every profile upload, rather than every other profile
upload.

Describe how to test/QA your changes

The unit test has been updated. Previously, with a profile period of 1 second
and a trace period of 3 seconds, it took 5 uploads to see the expected 2
traces. But really it should only have taken 4.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 0bda85e in PR branch nick.ripley/fix-exec-trace-timing with baseline commit 0614a05 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.

[felixge]

LGTM. 🙇