feat(metrics): Add cache metrics

src/Lavalink4NET.Rest/CacheAccessor.cs

@@ -1,34 +1,114 @@
 namespace Lavalink4NET.Rest;
 
 using System.Diagnostics.CodeAnalysis;
+using System.Diagnostics.Metrics;
 using Lavalink4NET.Rest.Entities;
 using Microsoft.Extensions.Caching.Memory;
 
 internal readonly record struct CacheAccessor<T>(IMemoryCache MemoryCache, string Key, CacheMode Mode)
 {
     public bool TryGet([MaybeNullWhen(false)] out T value)
     {
+        var metricTag = KeyValuePair.Create<string, object?>("Key", Key);
+        Diagnostics.CacheQueries.Add(1, metricTag);
+
         if (Mode is not CacheMode.Dynamic and not CacheMode.CacheOnly)
         {
+            Diagnostics.CacheQueryInvalidations.Add(1, metricTag);
+
             value = default;
             return false;
         }
 
         var result = MemoryCache.TryGetValue(Key, out value);
 
-        if (!result && Mode is CacheMode.CacheOnly)
+        if (!result)
         {
-            throw new InvalidOperationException("The item is not present in cache.");
+            if (Mode is CacheMode.CacheOnly)
+            {
+                Diagnostics.CacheHardFaults.Add(1, metricTag);
+                throw new InvalidOperationException("The item is not present in cache.");
+            }
+
+            Diagnostics.CacheSoftFaults.Add(1, metricTag);
+        }
+        else
+        {
+            Diagnostics.CacheHits.Add(1, metricTag);
         }
 
         return result;
     }
 
     public void Set(T value, TimeSpan relativeExpiration)
     {
+        var metricTag = KeyValuePair.Create<string, object?>("Key", Key);
+
         if (Mode is CacheMode.Dynamic or CacheMode.Refresh)
         {
+            Diagnostics.CacheUpdates.Add(1, metricTag);
             MemoryCache.Set(Key, value, relativeExpiration);
         }
+        else
+        {
+            Diagnostics.CacheUpdateInvalidations.Add(1, metricTag);
+        }
     }
 }
+
+file static class Diagnostics
+{
+    public static Counter<long> CacheHits { get; }
+
+    public static Counter<long> CacheSoftFaults { get; }
+
+    public static Counter<long> CacheHardFaults { get; }
+
+    public static Counter<long> CacheQueries { get; }
+
+    public static Counter<long> CacheUpdates { get; }
+
+    public static Counter<long> CacheQueryInvalidations { get; }
+
+    public static Counter<long> CacheUpdateInvalidations { get; }
+
+    static Diagnostics()
+    {
+        var meter = new Meter("Lavalink4NET");
+
+        CacheHits = meter.CreateCounter<long>(
+            name: "cache-hits",
+            unit: "Items",
+            description: "The number of items that were retrieved from the cache.");
+
+        CacheSoftFaults = meter.CreateCounter<long>(
+            name: "cache-soft-faults",
+            unit: "Items",
+            description: "The number of queries that could not be retrieved from the cache.");
+
+        CacheHardFaults = meter.CreateCounter<long>(
+            name: "cache-hard-faults",
+            unit: "Items",
+            description: "The number of queries which are specified as required that could not be retrieved from the cache.");
+
+        CacheQueries = meter.CreateCounter<long>(
+            name: "cache-queries",
+            unit: "Items",
+            description: "The number of queries to the cache.");
+
+        CacheUpdates = meter.CreateCounter<long>(
+            name: "cache-updates",
+            unit: "Items",
+            description: "The number of items that were added to the cache.");
+
+        CacheQueryInvalidations = meter.CreateCounter<long>(
+            name: "cache-query-invalidations",
+            unit: "Items",
+            description: "The number of queries that specified to bypass the cache.");
+
+        CacheUpdateInvalidations = meter.CreateCounter<long>(
+            name: "cache-update-invalidations",
+            unit: "Items",
+            description: "The number of updates that specified to bypass the cache.");
+    }
+}