feat: Implement statistics class & additional metrics

The new module/class helps us constructing queries and count records to
expose various statistics on the homepage. We also utilize for some new
prometheus metrics (package and user gauges).
Record counts are being cached with Redis.

Signed-off-by: moson <moson@archlinux.org>

aurweb/prometheus.py

@@ -1,6 +1,6 @@
 from typing import Any, Callable, Optional
 
-from prometheus_client import Counter
+from prometheus_client import Counter, Gauge
 from prometheus_fastapi_instrumentator import Instrumentator
 from prometheus_fastapi_instrumentator.metrics import Info
 from starlette.routing import Match, Route
@@ -11,10 +11,26 @@ logger = aur_logging.get_logger(__name__)
 _instrumentator = Instrumentator()
 
 
+# Custom metrics
+SEARCH_REQUESTS = Counter(
+    "aur_search_requests", "Number of search requests by cache hit/miss", ["cache"]
+)
+USERS = Gauge(
+    "aur_users", "Number of AUR users by type", ["type"], multiprocess_mode="livemax"
+)
+PACKAGES = Gauge(
+    "aur_packages",
+    "Number of AUR packages by state",
+    ["state"],
+    multiprocess_mode="livemax",
+)
+
+
 def instrumentator():
     return _instrumentator
 
 
+# FastAPI metrics
 # Taken from https://github.com/stephenhillier/starlette_exporter
 # Their license is included in LICENSES/starlette_exporter.
 # The code has been modified to remove child route checks