[ananthsub]

What does this PR do?

With traces disabled, setup_telemetry now enables no span group, so managed_span and trace_fn sites return before touching OpenTelemetry instead of building spans that no exporter will receive.

Before this change, a process with enabled=True and traces_enabled=False still stored the span-group spec. Every enabled group passed the gate in managed_span and trace_fn, so each gated site started a span on the no-op tracer, attached it to the current context, set its attributes, and ended it. That is a per-call cost on the hot path of every metrics-only process, in every consuming library, for spans that are discarded.

setup_telemetry now pins the empty set when traces_enabled is false, the same way it already does when telemetry is disabled. Pinning drops the stored spec, so a library that registers its groups after setup cannot enable one either. With traces enabled, nothing changes.

Issues

No issue. Found while measuring metrics-only overhead in NeMo Gym, where the same gap made Gym skip span-only work itself (NVIDIA-NeMo/Gym#4481).

A consumer that checks is_span_group_enabled before recording a metric would stop recording that metric in metrics-only runs once this lands. NeMo Gym did that for its verification, rollout, and sandbox metrics, and NVIDIA-NeMo/Gym#4612 moves those metrics onto the metrics-exporter check. NVIDIA-NeMo/Gym#4612 should merge before Gym bumps its nemo-lens pin past this change.

Usage

No API change. A metrics-only setup now behaves like this:

from nemo.lens import NemoLensConfig, enabled_span_groups, managed_span, setup_telemetry

setup_telemetry(NemoLensConfig(enabled=True, traces_enabled=False, span_groups="all", exporter="console"))
assert enabled_span_groups() == frozenset()
with managed_span("step", "train.step") as span:
    assert span is None  # the body runs; no span is created

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

• No public symbol or signature changes, so the consumer _fallbacks.py files need no edit.
• tests/test_handle.py covers the new behavior: with traces disabled no group is enabled and managed_span yields None, and a library that registers after setup stays disabled. Both tests fail without the change.
• docs/user-guide/configuration.mdx documents the effect of traces_enabled on span groups, and docs/user-guide/troubleshooting.mdx notes that enabled_span_groups() is empty while traces are disabled. README.md does not describe traces_enabled.
• Validated against NeMo Gym: Gym's unit suite on fix(telemetry): record metrics whenever metrics export, independent of span groups Gym#4612 runs the same against this branch as against the current pin.

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-87-4cb79c447970.docs.buildwithfern.com/nemo/lens

[ahmadki]

/ok to test 5e29619