Add Prometheus stats for admin count (#26284)

* Add Prometheus stats for admin count

Fixes #20828

Reports time series for admin count. Counts are separated by state (active, AFK, or deadminned) and admin rank.

* Use static constructor instead of static readonly for the metric

Docs recommend this due to inconsistent execution of C# static constructors.

* Remove static usage, use IoC IMeterFactory.

---------

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/Administration/Managers/AdminManager.cs

@@ -23,7 +23,7 @@ using Robust.Shared.Utility;
 
 namespace Content.Server.Administration.Managers
 {
-    public sealed class AdminManager : IAdminManager, IPostInjectInit, IConGroupControllerImplementation
+    public sealed partial class AdminManager : IAdminManager, IPostInjectInit, IConGroupControllerImplementation
     {
         [Dependency] private readonly IPlayerManager _playerManager = default!;
         [Dependency] private readonly IServerDbManager _dbManager = default!;
@@ -34,6 +34,7 @@ namespace Content.Server.Administration.Managers
         [Dependency] private readonly IServerConsoleHost _consoleHost = default!;
         [Dependency] private readonly IChatManager _chat = default!;
         [Dependency] private readonly ToolshedManager _toolshed = default!;
+        [Dependency] private readonly ILogManager _logManager = default!;
 
         private readonly Dictionary<ICommonSession, AdminReg> _admins = new();
         private readonly HashSet<NetUserId> _promotedPlayers = new();
@@ -49,6 +50,8 @@ namespace Content.Server.Administration.Managers
         private readonly AdminCommandPermissions _commandPermissions = new();
         private readonly AdminCommandPermissions _toolshedCommandPermissions = new();
 
+        private ISawmill _sawmill = default!;
+
         public bool IsAdmin(ICommonSession session, bool includeDeAdmin = false)
         {
             return GetAdminData(session, includeDeAdmin) != null;
@@ -181,6 +184,8 @@ namespace Content.Server.Administration.Managers
 
         public void Initialize()
         {
+            _sawmill = _logManager.GetSawmill("admin");
+
             _netMgr.RegisterNetMessage<MsgUpdateAdminStatus>();
 
             // Cache permissions for loaded console commands with the requisite attributes.
@@ -234,6 +239,8 @@ namespace Content.Server.Administration.Managers
             }
 
             _toolshed.ActivePermissionController = this;
+
+            InitializeMetrics();
         }
 
         public void PromoteHost(ICommonSession player)