profiler: increase timestamp precision

[felixge]

(low priority)

What does this PR do?

Change event.json timestamp precision from seconds to nanoseconds.

Motivation

The current precision limitations were discovered while playing with prof-tester locally and seeing a profile that should have been 1s reported as 2s due to rounding effects caused by the limited precision.

There is probably no immediate benefit for improving the precision for normal 60s+ profiles.

Describe how to test/QA your changes

go test ./profiler

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 0f34cd4 in PR branch felix.geisendoerfer/profiler-timestamp-precision with baseline commit 5ff4c5c in branch main.

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