Move all non-OTel tracing plumbing (Span/TracingProvider protocols, Noop
implementation, SpanName, registry, YDB attribute helpers) into a new
ydb.observability package. The OTel bridge in ydb.opentelemetry becomes a
concrete provider (OtelTracingProvider) that plugs into the same interface.

Users can now enable tracing without any OpenTelemetry dependency by
supplying a custom TracingProvider to ydb.observability.enable_tracing. A
second enable call always replaces the previously installed provider.

ydb.opentelemetry.tracing is kept as a thin re-export shim for backward
compatibility. All SDK internal imports (retries, connection, pool,
session, transaction — sync + aio) now go through ydb.observability.tracing.

Added tests/tracing/test_observability_enable.py covering: default Noop
state, custom provider wiring, reset semantics on double enable, disable
reverting to Noop, OTel enable replacing a custom provider, and duck-typed
providers satisfying the protocol.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

  - TestSplitEndpoint — parametric test covering every branch of _split_endpoint (grpcs://, grpc://, bare host:port, valid/malformed IPv6, missing colon, non-numeric port, None).
  - TestBuildYdbAttrsPeerOptionals — peer tuples with only address / only port / only location / empty location, hitting the three if x is not None branches in _build_ydb_attrs.
  - TestSetPeerAttributes — direct unit test for the set_peer_attributes helper (previously only reached through session flows that never had a non-None peer).
  - TestSpanFinishCallback — success and exception paths of the streaming _finish callback.
  - TestOpentelemetryTracingShimReexports — proves the ydb.opentelemetry.tracing back-compat shim re-exports the exact same objects as ydb.observability.tracing.
  - TestOtelEntrypointHandlesMissingPackage — uses sys.modules["ydb.opentelemetry.plugin"] = None to force the ImportError branches in ydb.opentelemetry.__init__.enable_tracing / disable_tracing.
  - TestOtelTracingSpanBridge — direct tests for TracingSpan.set_attribute and the defensive "exit before enter" branch in _AttachContext.
  - TestNoopContextManager — smoke tests for the Noop context manager and NoopTracingProvider returning the shared span + empty metadata.

…ability docs

- Expose OtelTracingProvider from ydb.opentelemetry via lazy __getattr__ so
  the documented example imports without pulling opentelemetry at load time.
- enable_tracing requires a provider (drop the None-as-disable overload);
  disable_tracing stays the single off switch.
- Type fixes: headers as Dict[str, str], Span.attach_context -> ContextManager.
- Docs: promote Observability to the umbrella page (interface, custom providers,
  the streaming end_on_exit contract); OpenTelemetry becomes a backend section.

…is on

While a tracing provider is installed the SDK appends ydb-sdk-tracing/0.1.0 to
the x-ydb-sdk-build-info header (native SDK, then SDK-owned feature tokens, then
custom driver headers). The token list is aggregated by
ydb.observability.sdk_build_info_tokens so future features (e.g. metrics) plug in
with a single line.

…tr__

Module __getattr__ raising ImportError broke hasattr()/getattr(default)
introspection of OtelTracingProvider when OpenTelemetry is absent. Raise
AttributeError instead, keeping the install hint in the message; enable_tracing()
still surfaces its own ImportError guidance.