Add timer to sending-metrics-sdk (#1089)

* Add timer to sending-metrics-sdk

Documenting missing capability/example to send timers

* Update sending-metrics-sdk.mdx

---------

Co-authored-by: Armin Ronacher <armin.ronacher@active-4.com>

src/docs/delightful-developer-metrics/sending-metrics-sdk.mdx

@@ -70,6 +70,19 @@ metrics.gauge(
     tags=tags,
 )
 ```
+If you want to measure how much time a specific piece of code takes, you can use:
+
+```python
+from sentry_sdk import metrics
+
+# Emit a distribution metric of the execution time of the function.
+with metrics.timer("my_timer"):
+    pass
+
+@metrics.timer("my_timer")
+def my_function():
+    pass
+```
 
 <Alert title="Note" level="note">
 As of now, only the Python SDK is supporting the new metrics features but we are working on supporting metrics in the JavaScript and Rust SDKs next.