[Misc] Fix unintentional truncation of milliseconds in metric recordings
Description
Makes Timer::msecs return a double instead of an int64_t to ensure metric recording of <1 msec don't fall into the wrong bucket.
Checklist
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Documentation reflects the changes made. -
Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs
References
Closes #1429 (closed)