add restricted registry documentation (#968)

* add restricted registry documentation
* show example for generate_latest

Signed-off-by: remi <remijouannet@gmail.com>
Signed-off-by: Remi Jouannet <remijouannet@gmail.com>
Co-authored-by: Chris Marchbanks <csmarchbanks@gmail.com>

README.md

@@ -737,6 +737,34 @@ for family in text_string_to_metric_families(u"my_gauge 1.0\n"):
     print("Name: {0} Labels: {1} Value: {2}".format(*sample))
 ```
 
+## Restricted registry
+
+Registries support restriction to only return specific metrics.
+If you’re using the built-in HTTP server, you can use the GET parameter "name[]", since it’s an array it can be used multiple times.
+If you’re directly using `generate_latest`, you can use the function `restricted_registry()`.
+
+```python
+curl --get --data-urlencode "name[]=python_gc_objects_collected_total" --data-urlencode "name[]=python_info" http://127.0.0.1:9200/metrics
+```
+
+```python
+from prometheus_client import generate_latest
+
+generate_latest(REGISTRY.restricted_registry(['python_gc_objects_collected_total', 'python_info']))
+```
+
+```python
+# HELP python_info Python platform information
+# TYPE python_info gauge
+python_info{implementation="CPython",major="3",minor="9",patchlevel="3",version="3.9.3"} 1.0
+# HELP python_gc_objects_collected_total Objects collected during gc
+# TYPE python_gc_objects_collected_total counter
+python_gc_objects_collected_total{generation="0"} 73129.0
+python_gc_objects_collected_total{generation="1"} 8594.0
+python_gc_objects_collected_total{generation="2"} 296.0
+```
+
+
 ## Links
 
 * [Releases](https://github.com/prometheus/client_python/releases): The releases page shows the history of the project and acts as a changelog.