[Feature]: populate `session_started_ns` correctly

[arekay-nv]

Motivation

Currently, session_started_ns is not populated and left as a TODO item.

Proposed Solution

Populate it with the benchmark start time ideally right before performance phase starts.
Need to ensure that it is not inconsistent with other reporting metrics (check if we already publish any timestamps ).

Alternatives Considered

No response

Additional Context

No response

[anandhu-eng]

Thanks for creating the issue @arekay-nv .

Suggestion: Storing the value in any of the output json/yaml file could make the downstream parsing simpler in submission infra