Copybara import of the project:

[copybara-service]

Copybara import of the project:

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23ee67c by Yu Yi yiyu@google.com:

Add opt-in OpenTelemetry double-write to MetricsLogger

Add an experimental, off-by-default enable_opentelemetry flag to
MetricsLoggerOptions. When set, every scalar logged through
MetricsLogger.log() is emitted both through the existing
Metrax/jax.monitoring backends (unchanged, still the default) and as
OpenTelemetry gauges with stable tunix.* instrument names,
low-cardinality prefix/mode attributes, and a separate
tunix.training.step gauge.

Because Weights & Biases has no OTLP endpoint for run metrics (its
native OTLP ingestion is traces-only via Weave), also add
tunix.sft.otel_wandb.WandbMetricsExporter, an OpenTelemetry SDK metric
exporter that forwards the gauges to wandb.log using the legacy
{prefix}/{mode}/{name} chart keys so both pipelines can be compared in
the same W&B project. An offline end-to-end test exercises the real
wandb client (wandb.init(mode="offline"), no account or network
needed) and verifies the per-step history written to the run.

OpenTelemetry is a new optional [otel] extra, not a core dependency;
providers and exporters remain application-owned.

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