feat(common): telemetry module for optional OpenTelemetry instrumentation`

[aashvgit]

What this PR does / why we need it/summary:
Adds kubeflow/common/telemetry.py — the shared foundation module for
optional OpenTelemetry instrumentation across all Kubeflow SDK clients.

This is a draft PR for GSoC 2026 Issue for project 7
Design Decisions
Zero overhead when disabled: get_tracer() returns a _NoOpTracer
when opentelemetry-api is not installed. No import errors, no exceptions.

Opt-out via env var: KUBEFLOW_TRACING_DISABLED=1 disables all
instrumentation with no code changes required.

Configurable exporters:Supports console, otlp, and none via
configure(exporter=...). OTLP endpoint configurable via env var
KUBEFLOW_OTLP_ENDPOINT.

Consistent naming: SpanNames and SpanAttributes constants enforce
naming across all clients. Span names follow kubeflow.sdk.<client>.<op>
convention. Attributes follow OTel semantic conventions for GenAI workload.

Sampling: Configurable via sample_rate parameter f(0.0–1.0).
** Span Hierarchy (taking TrainerClient as example)**

kubeflow.sdk.trainer.train
kubeflow.sdk.trainer.create_trainjob [event: trainjob_submitted]
kubeflow.sdk.trainer.poll_status [× N per polling]
attributes: poll.iteration, kubeflow.trainer.status

Tests
10 unit tests following TestCase + pytest.mark.parametrize pattern, All tests pass without opentelemetry-api installed (no-op path).

Fixes #
#164 as this is the foundation model
Checklist:

• Docs included if any changes are user facing

[github-actions]

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