feat(tracing): Add types for traces_sampler implementation (#864)

- Types for the `traces_sampler` itself (the function and its input)
- A new attribute on the `Transaction` class tracking the parent sampling decision separately from the sampling decision of the transaction itself, since part of the `traces_sampler` spec is that there needs to be a difference between an inherited decision and an explicitly set decision.

sentry_sdk/_types.py

@@ -5,13 +5,15 @@
 
 
 if MYPY:
+    from numbers import Real
     from types import TracebackType
     from typing import Any
     from typing import Callable
     from typing import Dict
     from typing import Optional
     from typing import Tuple
     from typing import Type
+    from typing import Union
     from typing_extensions import Literal
 
     ExcInfo = Tuple[
@@ -24,10 +26,14 @@
     Breadcrumb = Dict[str, Any]
     BreadcrumbHint = Dict[str, Any]
 
+    SamplingContext = Dict[str, Any]
+
     EventProcessor = Callable[[Event, Hint], Optional[Event]]
     ErrorProcessor = Callable[[Event, ExcInfo], Optional[Event]]
     BreadcrumbProcessor = Callable[[Breadcrumb, BreadcrumbHint], Optional[Breadcrumb]]
 
+    TracesSampler = Callable[[SamplingContext], Union[Real, bool]]
+
     # https://github.com/python/mypy/issues/5710
     NotImplementedType = Any
 

sentry_sdk/consts.py

@@ -14,7 +14,12 @@
     from sentry_sdk.transport import Transport
     from sentry_sdk.integrations import Integration
 
-    from sentry_sdk._types import Event, EventProcessor, BreadcrumbProcessor
+    from sentry_sdk._types import (
+        BreadcrumbProcessor,
+        Event,
+        EventProcessor,
+        TracesSampler,
+    )
 
     # Experiments are feature flags to enable and disable certain unstable SDK
     # functionality. Changing them from the defaults (`None`) in production
@@ -65,6 +70,7 @@ def __init__(
         ca_certs=None,  # type: Optional[str]
         propagate_traces=True,  # type: bool
         traces_sample_rate=0.0,  # type: float
+        traces_sampler=None,  # type: Optional[TracesSampler]
         auto_enabling_integrations=True,  # type: bool
         _experiments={},  # type: Experiments  # noqa: B006
     ):

sentry_sdk/tracing.py

@@ -449,11 +449,12 @@ def get_trace_context(self):
 
 
 class Transaction(Span):
-    __slots__ = ("name",)
+    __slots__ = ("name", "parent_sampled")
 
     def __init__(
         self,
         name="",  # type: str
+        parent_sampled=None,  # type: Optional[bool]
         **kwargs  # type: Any
     ):
         # type: (...) -> None
@@ -468,6 +469,7 @@ def __init__(
             name = kwargs.pop("transaction")
         Span.__init__(self, **kwargs)
         self.name = name
+        self.parent_sampled = parent_sampled
 
     def __repr__(self):
         # type: () -> str