feat(bot): add stats tracking for dm channel cache hits/misses

PluralKit.Bot/BotMetrics.cs

@@ -22,6 +22,27 @@ public static class BotMetrics
         Context = "Bot"
     };
 
+    public static MeterOptions LocalDMCacheHits => new()
+    {
+        Name = "Cluster local DM Cache Hits",
+        MeasurementUnit = Unit.Calls,
+        Context = "Bot"
+    };
+
+    public static MeterOptions DatabaseDMCacheHits => new()
+    {
+        Name = "Database DM Cache Hits",
+        MeasurementUnit = Unit.Calls,
+        Context = "Bot"
+    };
+
+    public static MeterOptions DMCacheMisses => new()
+    {
+        Name = "DM Cache Misses",
+        MeasurementUnit = Unit.Calls,
+        Context = "Bot"
+    };
+
     public static MeterOptions CommandsRun => new()
     {
         Name = "Commands run",

PluralKit.Bot/Services/PrivateChannelService.cs

@@ -1,3 +1,5 @@
+using App.Metrics;
+
 using Serilog;
 
 using Myriad.Gateway;
@@ -11,11 +13,13 @@ public class PrivateChannelService
 {
     private static readonly Dictionary<ulong, ulong> _channelsCache = new();
 
+    private readonly IMetrics _metrics;
     private readonly ILogger _logger;
     private readonly ModelRepository _repo;
     private readonly DiscordApiClient _rest;
-    public PrivateChannelService(ILogger logger, ModelRepository repo, DiscordApiClient rest)
+    public PrivateChannelService(IMetrics metrics, ILogger logger, ModelRepository repo, DiscordApiClient rest)
     {
+        _metrics = metrics;
         _logger = logger;
         _repo = repo;
         _rest = rest;
@@ -32,16 +36,23 @@ public class PrivateChannelService
     public async Task<ulong> GetOrCreateDmChannel(ulong userId)
     {
         if (_channelsCache.TryGetValue(userId, out var cachedChannelId))
+        {
+            _metrics.Measure.Meter.Mark(BotMetrics.LocalDMCacheHits);
             return cachedChannelId;
+        }
 
         var channelId = await _repo.GetDmChannel(userId);
         if (channelId == null)
         {
+            _metrics.Measure.Meter.Mark(BotMetrics.DMCacheMisses);
             var channel = await _rest.CreateDm(userId);
             channelId = channel.Id;
         }
 
+        _metrics.Measure.Meter.Mark(BotMetrics.DatabaseDMCacheHits);
+
         // spawn off saving the channel as to not block the current thread
+        // todo: don't save to database again if we just fetched it from there
         _ = SaveDmChannel(userId, channelId.Value);
 
         return channelId.Value;