feat(trace): add explicit span emission helpers

[rhewett-nv]

What does this PR do?

Adds explicit timestamp span utilities for emitting completed spans and deriving Linux process start time without expanding the top-level Lens API.

Issues

N/A

Usage

import time

from opentelemetry import trace

from nemo.lens.span_utilities import emit_span, linux_process_create_time

tracer = trace.get_tracer("nv.dl.launch")

process_start = linux_process_create_time()
workload_start = time.time()

emit_span(
    tracer,
    "nv.dl.launch.container_load",
    process_start,
    workload_start,
    attributes={"example.phase": "startup"},
)

Before your PR is "Ready for review"

Pre checks:

• I read the Contributor guidelines.
• All commits are signed off (git commit -s) per the DCO.
• I added or updated tests under tests/ for any behavior change.
• I ran pytest and pre-commit run --all-files locally; both pass.
• I updated docs/ and README.md where behavior, configuration, or public API changed.
• If this changes a lens public API symbol, I updated the cross-repo _fallbacks.py files in Megatron-LM / RL / Gym.

Additional information

Validation on pr2-span-utilities:

• ../.venv/bin/pre-commit run --all-files: passed
• PYTHONPATH=src ../.venv/bin/python -m pytest -q: 247 passed, 7 warnings
• ../.venv/bin/python -m compileall -q src tests: passed
• npm --prefix docs/fern run check: passed with 0 errors; one warning because the unauthenticated redirects check is skipped without fern login / FERN_TOKEN

Notes:

• PR2 is one signed commit: 1828ae4 feat(trace): add explicit span emission helpers.
• It adds src/nemo/lens/span_utilities.py and tests/test_span_utilities.py.
• It does not add top-level nemo.lens exports.
• It adds no opentelemetry.sdk imports under src/; SDK usage is test-only.
• No cross-repo fallback update is required because this does not expand the fallback surface.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[ahmadki]

/ok to test 1828ae4

[ahmadki]

/ok to test 66d2c09