[SWE] Make Gym startup metrics optional and atomic

[seonjinn]

Problem

We need to understand where SWE rollout startup time is spent: connecting to NeMo Gym, creating the runtime, or initializing the agent.

The existing timers write into a NeMo Gym metrics file. This makes standalone OpenHands runs fragile when the file is not configured.

High-level change

This PR makes the existing startup timers safe and optional.

NeMo Gym run:     measure startup phases -> update the metrics file
Standalone run:   no metrics file        -> continue normally

It keeps the existing phase boundaries and timer names. It only changes how their results are published:

• metrics are enabled only when NEMO_GYM_METRICS_FPATH is set;
• standalone OpenHands runs cleanly skip the metrics write;
• existing metric values are preserved; and
• each JSON update is written atomically.

Expected impact

This is an observability change, not a rollout speedup by itself. It gives us reliable phase-level data needed to find and validate Init and Framework overhead reductions.

It does not change agent commands, retries, controller behavior, or runtime setup.

Change Type

• Bug fix
• New feature
• Breaking change
• Refactor
• Other

Release Notes

• Include this change in the Release Notes.