Split tracing into vendor-neutral observability + OTel adapter

[KirillKurdyukov]

Summary

Fixes an architectural issue: tracing was tightly coupled to OpenTelemetry — the SDK forced users to have opentelemetry installed just to enable tracing, and there was no way to plug in a custom backend.

This PR splits the module in two:

• ydb.observability (new) — vendor-neutral interfaces (Span, TracingProvider), Noop implementation, SpanName, registry, and all SDK-side helpers (create_ydb_span, set_peer_attributes, span_finish_callback, get_trace_metadata). The SDK core imports only from here and never touches opentelemetry.
• ydb.opentelemetry — becomes a concrete OtelTracingProvider that plugs into the same interface. ydb.opentelemetry.enable_tracing(tracer=None) is now a thin convenience that builds the provider and hands it to ydb.observability.enable_tracing(...).

New public API

from ydb.observability import Span, TracingProvider, enable_tracing, disable_tracing

class MyProvider:
    def create_span(self, name, attributes=None, kind=None) -> Span: ...
    def get_trace_metadata(self): ...

enable_tracing(MyProvider())   # any provider works, no OTel required
enable_tracing(MyProvider())   # second call *replaces* the previous provider
disable_tracing()              # back to Noop

• Double-enable_tracing cleanly resets the previously installed provider (works for OTel and custom providers alike — previously ydb.opentelemetry.enable_tracing was silently idempotent).
• ydb.opentelemetry.tracing remains as a re-export shim for backward compatibility.

Tests

• All 55 existing tracing tests pass (55/55).
• New tests/tracing/test_observability_enable.py (12 tests) uses a hand-rolled RecordingProvider — no OTel involvement — to cover:
  • default Noop state / empty metadata,
  • custom provider receiving spans and metadata,
  • reset on double enable_tracing,
  • enable_tracing(None) == disable_tracing(),
  • disable_tracing reverts to Noop,
  • ydb.opentelemetry.enable_tracing replacing a custom provider,
  • duck-typed provider satisfying the TracingProvider protocol.

Test plan

• tox -e black-format + tox -e black
• tox -e style
• tox -e mypy
• tox -e py -- ydb -v (174 passed)
• tox -e py -- tests/tracing -v (55 passed, including 12 new observability tests)
• Full integration suite (requires Docker; local run)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.55%. Comparing base (a7bf659) to head (d6aed82).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #861      +/-   ##
==========================================
+ Coverage   81.38%   81.55%   +0.17%     
==========================================
  Files          94       96       +2     
  Lines       12197    12209      +12     
  Branches     1204     1201       -3     
==========================================
+ Hits         9926     9957      +31     
+ Misses       1807     1799       -8     
+ Partials      464      453      -11     

Flag	Coverage Δ
integration	79.35% <100.00%> (+0.20%)	⬆️
unit	47.31% <47.26%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
ydb/aio/connection.py	88.81% <100.00%> (ø)
ydb/aio/pool.py	75.21% <100.00%> (ø)
ydb/aio/query/session.py	88.60% <100.00%> (ø)
ydb/aio/query/transaction.py	100.00% <100.00%> (ø)
ydb/connection.py	80.46% <100.00%> (ø)
ydb/observability/__init__.py	100.00% <100.00%> (ø)
ydb/observability/tracing.py	100.00% <100.00%> (ø)
ydb/opentelemetry/__init__.py	100.00% <ø> (+30.76%)	⬆️
ydb/opentelemetry/plugin.py	100.00% <100.00%> (+5.88%)	⬆️
ydb/opentelemetry/tracing.py	100.00% <100.00%> (+10.67%)	⬆️
... and 4 more

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