ref: Add new_trace, continue_trace to span first (6) (#5593)

### Description
Add the new `sentry_sdk.traces.continue_trace` and
`sentry_sdk.traces.new_trace` APIs.

#### Issues
<!--
* resolves: #1234
* resolves: LIN-1234
-->

#### Reminders
- Please add tests to validate your changes, and lint your code using
`tox -e linters`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

Author: sentrivana

sentry_sdk/traces.py

@@ -14,7 +14,7 @@
 from sentry_sdk.utils import format_attribute, logger
 
 if TYPE_CHECKING:
-    from typing import Optional, Union
+    from typing import Any, Optional, Union
     from sentry_sdk._types import Attributes, AttributeValue
 
 
@@ -99,6 +99,9 @@ def start_span(
     span.end()
     ```
 
+    To continue a trace from another service, call
+    `sentry_sdk.traces.continue_trace()` prior to creating a top-level span.
+
     :param name: The name to identify this span by.
     :type name: str
 
@@ -126,6 +129,44 @@ def start_span(
     )
 
 
+def continue_trace(incoming: "dict[str, Any]") -> None:
+    """
+    Continue a trace from headers or environment variables.
+
+    This function sets the propagation context on the scope. Any span started
+    in the updated scope will belong under the trace extracted from the
+    provided propagation headers or environment variables.
+
+    continue_trace() doesn't start any spans on its own. Use the start_span()
+    API for that.
+    """
+    # This is set both on the isolation and the current scope for compatibility
+    # reasons. Conceptually, it belongs on the isolation scope, and it also
+    # used to be set there in non-span-first mode. But in span first mode, we
+    # start spans on the current scope, regardless of type, like JS does, so we
+    # need to set the propagation context there.
+    sentry_sdk.get_isolation_scope().generate_propagation_context(
+        incoming,
+    )
+    sentry_sdk.get_current_scope().generate_propagation_context(
+        incoming,
+    )
+
+
+def new_trace() -> None:
+    """
+    Resets the propagation context, forcing a new trace.
+
+    This function sets the propagation context on the scope. Any span started
+    in the updated scope will start its own trace.
+
+    new_trace() doesn't start any spans on its own. Use the start_span() API
+    for that.
+    """
+    sentry_sdk.get_isolation_scope().set_new_propagation_context()
+    sentry_sdk.get_current_scope().set_new_propagation_context()
+
+
 class StreamedSpan:
     """
     A span holds timing information of a block of code.