Added docs for "functions_to_trace" option. (#6520)

Co-authored-by: Liza Mock <[email protected]>

Author: antonpirker

src/platforms/common/configuration/options.mdx

@@ -775,6 +775,17 @@ If <PlatformIdentifier name="trace-propagation-targets" /> is not provided, trac
 
 </ConfigKey>
 
+<ConfigKey name="functions-to-trace" supported={["python"]}>
+
+An optional list of functions that should be set up for performance monitoring. For each function in the list, a span will be created when the function is executed.
+Functions in the list are represented as strings containing the fully qualified name of the fuction.
+
+This is a convenient option, making it possible to have one central place for configuring what functions to trace, instead of having manual instrumentation scattered all over your code base.
+
+To learn more, see the [Custom Instrumentation](/platforms/python/performance/instrumentation/custom-instrumentation/#define-span-creation-in-a-central-place) documentation.
+
+</ConfigKey>
+
 <ConfigKey name="trace-options-requests" supported={["java"]}>
 
 <PlatformSection supported={["java"]}>

src/platforms/python/common/performance/instrumentation/custom-instrumentation.mdx

@@ -8,7 +8,7 @@ The Sentry SDK for Python does a very good job of auto instrumenting your applic
 
 ## Add a Transaction
 
-Adding transactions will allow you to instrument and capture certain regions of your code. 
+Adding transactions will allow you to instrument and capture certain regions of your code.
 
 <Note>
 
@@ -106,6 +106,35 @@ def eat_slice(slice):
     swallow()
 ```
 
+## Define Span Creation in a Central Place
+
+To avoid having custom performance instrumentation code scattered all over your code base, pass a parameter <PlatformIdentifier name="functions-to-trace" /> to your `sentry_sdk.init()` call.
+
+```python
+import sentry_sdk
+
+functions_to_trace = [
+    "myrootmodule.eat_slice",
+    "myrootmodule.swallow",
+    "myrootmodule.chew",
+    "myrootmodule.someothermodule.another.some_function",
+    "myrootmodule.SomePizzaClass.some_method",
+]
+
+sentry_sdk.init(
+    dsn="___PUBLIC_DSN___",
+    functions_to_trace=functions_to_trace,
+)
+```
+
+Now, whenever a function specified in `functions_to_trace` will be executed, a span will be created and attached as a child to the currently running span.
+
+<Alert level="warning" title="Important">
+
+To enable performance monitoring for the functions specified in `functions_to_trace`, the SDK needs to load the function modules. Be aware, there may be code being executed in modules during module loading. To avoid this, use the method described above to trace your functions.
+
+</Alert>
+
 ## Accessing the Current Transaction
 
 To change data in an already ongoing transaction, use `Hub.current.scope.transaction`. This property will return a transaction if there's one running, otherwise it will return `None`.