Fastapi + Gunicorn example in README

Also provided additional multiprocessing instructions for FastAPI + Gunicorn setup with code examples as per this issue: prometheus#810

Signed-off-by: Matas Minelga <minematas@gmail.com>

README.md

@@ -404,6 +404,56 @@ uwsgi --http 127.0.0.1:8000 --wsgi-file myapp.py --callable app
 
 Visit http://localhost:8000/metrics to see the metrics
 
+#### FastAPI + Gunicorn
+
+To use Prometheus with [FastAPI](https://fastapi.tiangolo.com/) and [Gunicorn](https://gunicorn.org/) we need to serve metrics through a Prometheus ASGI application.
+
+Save the snippet below in a `myapp.py` file
+
+```python
+from fastapi import FastAPI
+from prometheus_client import make_asgi_app
+
+# Create app
+app = FastAPI(debug=False)
+
+# Add prometheus asgi middleware to route /metrics requests
+metrics_app = make_asgi_app()
+app.mount("/metrics", metrics_app)
+```
+
+For Multiprocessing support, use this modified code snippet. Full multiprocessing intstructions are provided [here](https://github.com/prometheus/client_python#multiprocess-mode-eg-gunicorn).
+
+```python
+from fastapi import FastAPI
+from prometheus_client import make_asgi_app
+
+app = FastAPI(debug=False)
+
+# Using multiprocess collector for registry
+def make_metrics_app():
+    registry = CollectorRegistry()
+    multiprocess.MultiProcessCollector(registry)
+    return prometheus_client.make_asgi_app(registry=registry)
+
+
+metrics_app = make_metrics_app()
+app.mount("/metrics", metrics_app))
+
+```
+
+Run the example web application like this
+
+```bash
+# Install gunicorn if you do not have it
+pip install gunicorn
+# If using multiple workers, add `--workers n` parameter to the line below
+gunicorn -b 127.0.0.1:8000 myapp:app -k uvicorn.workers.UvicornWorker
+```
+
+Visit http://localhost:8000/metrics to see the metrics
+
+
 ### Node exporter textfile collector
 
 The [textfile collector](https://github.com/prometheus/node_exporter#textfile-collector)