PYTHON-5945 Cache the OpenTelemetry tracer instead of fetching it per command

trace.get_tracer() was being called on every command span. Even on an
SDK cache hit this allocates two objects, takes a process-wide lock,
and mutates the global warnings filter list, ~300x slower than a
cached attribute read. Caching at import time is safe because
get_tracer() returns a self-updating ProxyTracer when no real
TracerProvider is registered yet.

Author: blink1073

pymongo/_otel.py

@@ -35,8 +35,15 @@
     from opentelemetry.trace import SpanKind, Status, StatusCode
 
     _HAS_OPENTELEMETRY = True
+    # Safe to cache at import time: opentelemetry.trace.get_tracer() returns a
+    # ProxyTracer when no real TracerProvider is registered yet, and that proxy
+    # transparently starts delegating to the real tracer once the application
+    # calls trace.set_tracer_provider() later, so this doesn't bind us to a
+    # permanently-inert no-op tracer.
+    _TRACER: Optional[Tracer] = trace.get_tracer("PyMongo", __version__)
 except ImportError:
     _HAS_OPENTELEMETRY = False
+    _TRACER = None
 
 if TYPE_CHECKING:
     from opentelemetry.trace import Span, Tracer
@@ -99,11 +106,6 @@ def _is_tracing_enabled(tracing_options: Optional[TracingOptions]) -> bool:
     return _env_truthy(_OTEL_ENABLED_ENV)
 
 
-def _get_tracer() -> Tracer:
-    """Return a Tracer scoped to this driver's name and version."""
-    return trace.get_tracer("PyMongo", __version__)
-
-
 def _get_query_text_max_length(tracing_options: Optional[TracingOptions]) -> int:
     """Return the configured db.query.text truncation length, or 0 to omit the attribute.
 
@@ -236,8 +238,8 @@ def start_command_span(
     if max_query_text_length > 0:
         attributes["db.query.text"] = _build_query_text(cmd, max_query_text_length)
 
-    tracer = _get_tracer()
-    return tracer.start_span(command_name, kind=SpanKind.CLIENT, attributes=attributes)
+    assert _TRACER is not None  # _is_tracing_enabled already checked _HAS_OPENTELEMETRY
+    return _TRACER.start_span(command_name, kind=SpanKind.CLIENT, attributes=attributes)
 
 
 def end_command_span_success(span: Optional[Span], reply: _DocumentOut) -> None: