feat: Add FastAPI s6 sidecar PoC

- Sidecar DragonflyDB as Redis replacement
- Run pytheus in multiprocessing mode with Redis backend

app/main.py

@@ -0,0 +1,20 @@
+from fastapi import FastAPI
+from fastapi.responses import PlainTextResponse
+from pytheus.exposition import generate_metrics
+from pytheus.middleware import PytheusMiddlewareASGI
+
+from pytheus.backends import load_backend
+from pytheus.backends.redis import MultiProcessRedisBackend
+
+load_backend(
+    backend_class=MultiProcessRedisBackend,
+    backend_config={"host": "127.0.0.1", "port": 6379},
+)
+
+app = FastAPI()
+app.add_middleware(PytheusMiddlewareASGI)
+
+
+@app.get("/metrics", response_class=PlainTextResponse)
+async def pytheus_metrics():
+    return generate_metrics()