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.Metrics.cs

@@ -0,0 +1,98 @@
+using System.Diagnostics.Metrics;
+using System.Runtime.InteropServices;
+using Content.Server.Afk;
+using Robust.Server.DataMetrics;
+
+namespace Content.Server.Administration.Managers;
+
+// Handles metrics reporting for active admin count and such.
+
+public sealed partial class AdminManager
+{
+    private Dictionary<int, (int active, int afk, int deadminned)>? _adminOnlineCounts;
+
+    private const int SentinelRankId = -1;
+
+    [Dependency] private readonly IMetricsManager _metrics = default!;
+    [Dependency] private readonly IAfkManager _afkManager = default!;
+    [Dependency] private readonly IMeterFactory _meterFactory = default!;
+
+    private void InitializeMetrics()
+    {
+        _metrics.UpdateMetrics += MetricsOnUpdateMetrics;
+
+        var meter = _meterFactory.Create("SS14.AdminManager");
+
+        meter.CreateObservableGauge(
+            "admins_online_count",
+            MeasureAdminCount,
+            null,
+            "The count of online admins");
+    }
+
+    private void MetricsOnUpdateMetrics()
+    {
+        _sawmill.Verbose("Updating metrics");
+
+        var dict = new Dictionary<int, (int active, int afk, int deadminned)>();
+
+        foreach (var (session, reg) in _admins)
+        {
+            var rankId = reg.RankId ?? SentinelRankId;
+
+            ref var counts = ref CollectionsMarshal.GetValueRefOrAddDefault(dict, rankId, out _);
+
+            if (reg.Data.Active)
+            {
+                if (_afkManager.IsAfk(session))
+                    counts.afk += 1;
+                else
+                    counts.active += 1;
+            }
+            else
+            {
+                counts.deadminned += 1;
+            }
+        }
+
+        // Neither prometheus-net nor dotnet-counters seem to handle stuff well if we STOP returning measurements.
+        // i.e. if the last admin with a rank disconnects.
+        // So if we have EVER reported a rank, always keep reporting it.
+        if (_adminOnlineCounts != null)
+        {
+            foreach (var rank in _adminOnlineCounts.Keys)
+            {
+                CollectionsMarshal.GetValueRefOrAddDefault(dict, rank, out _);
+            }
+        }
+
+        // Make sure "no rank" is always available. Avoid "no data".
+        CollectionsMarshal.GetValueRefOrAddDefault(dict, SentinelRankId, out _);
+
+        _adminOnlineCounts = dict;
+    }
+
+    private IEnumerable<Measurement<int>> MeasureAdminCount()
+    {
+        if (_adminOnlineCounts == null)
+            yield break;
+
+        foreach (var (rank, (active, afk, deadminned)) in _adminOnlineCounts)
+        {
+            yield return new Measurement<int>(
+                active,
+                new KeyValuePair<string, object?>("state", "active"),
+                new KeyValuePair<string, object?>("rank", rank == SentinelRankId ? "none" : rank.ToString()));
+
+            yield return new Measurement<int>(
+                afk,
+                new KeyValuePair<string, object?>("state", "afk"),
+                new KeyValuePair<string, object?>("rank", rank == SentinelRankId ? "none" : rank.ToString()));
+
+            yield return new Measurement<int>(
+                deadminned,
+                new KeyValuePair<string, object?>("state", "deadminned"),
+                new KeyValuePair<string, object?>("rank", rank == SentinelRankId ? "none" : rank.ToString()));
+        }
+    }
+}