Started implementing metrics system

Moonlight.ApiServer/Implementations/Metrics/ApplicationMetric.cs

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+using System.Diagnostics.Metrics;
+using Moonlight.ApiServer.Interfaces;
+using Moonlight.ApiServer.Services;
+
+namespace Moonlight.ApiServer.Implementations.Metrics;
+
+public class ApplicationMetric : IMetric
+{
+    private Gauge<long> MemoryUsage;
+    private Gauge<int> CpuUsage;
+    private Gauge<double> Uptime;
+    
+    public Task Initialize(Meter meter)
+    {
+        MemoryUsage = meter.CreateGauge<long>("moonlight_memory_usage");
+        CpuUsage = meter.CreateGauge<int>("moonlight_cpu_usage");
+        Uptime = meter.CreateGauge<double>("moonlight_uptime");
+        
+        return Task.CompletedTask;
+    }
+
+    public async Task Run(IServiceProvider provider, CancellationToken cancellationToken)
+    {
+        var applicationService = provider.GetRequiredService<ApplicationService>();
+
+        var memory = await applicationService.GetMemoryUsage();
+        MemoryUsage.Record(memory);
+
+        var uptime = await applicationService.GetUptime();
+        Uptime.Record(uptime.TotalSeconds);
+
+        var cpu = await applicationService.GetCpuUsage();
+        CpuUsage.Record(cpu);
+    }
+}

Moonlight.ApiServer/Implementations/Metrics/UsersMetric.cs

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+using System.Diagnostics.Metrics;
+using Microsoft.EntityFrameworkCore;
+using MoonCore.Extended.Abstractions;
+using Moonlight.ApiServer.Database.Entities;
+using Moonlight.ApiServer.Interfaces;
+
+namespace Moonlight.ApiServer.Implementations.Metrics;
+
+public class UsersMetric : IMetric
+{
+    private Gauge<int> Users;
+    
+    public Task Initialize(Meter meter)
+    {
+        Users = meter.CreateGauge<int>("moonlight_users");
+        
+        return Task.CompletedTask;
+    }
+
+    public async Task Run(IServiceProvider provider, CancellationToken cancellationToken)
+    {
+        var usersRepo = provider.GetRequiredService<DatabaseRepository<User>>();
+        var count = await usersRepo.Get().CountAsync(cancellationToken: cancellationToken);
+        
+        Users.Record(count);
+    }
+}